A while ago I saw this article explaining why aliens built the pyramids.

While I can easily scream 'MATHS AND ASTRONOMY', flip a table and leave to prove them wrong, I don't think I'll do that now.

So one by one, I'll summarise the arguments, try to not get angry (I'm actually not sure if it's a joke.) and explain why it doesn't work that way.
It'll be hard to do this without diagrams (which is also why I can't be your maths tutor here, darnit), but here we go.
www. outerworlds likeness/aliens/aliens. html

Argument One:
Okay, first off, the article is on a purple background and uses lots of unnecessary caps and explanation marks/question marks, so this'll be painful.

Argument One: A pyramid (not sure which one) lies before the delta of the Nile, and when lines are drawn north-west and north-east, they neatly and comfortably encase the delta.
Is that a coincidence? The aliens obviously did this, because why would the Egyptians get someone to walk hundreds of miles with a mega-long string in one direction and get another to do the same on the opposite side of the delta? Why the hell would they do that just to build a big building? Is that likely?
The aliens obviously did that.

Rebuttal: If you look at the diagram and draw a line straight through the middle, you'll see the lines are around 45 degrees away from it, which means the delta's around 90 degrees. So anyway, that's kind of unrelated, but it's important to know that the ancient Egyptians were far from primitive and I wouldn't put it past them to figure out a way to do that that involves, like… stars or something. Also, they had kickass mathematicians.
As for why it was such a big-ass building, this is the Ancient Egyptians we're talking about. The pharaohs loved them some pyramids.

Argument Two: There's this pyramid (again, not sure which one) and you'll see that it lines up exactly with the magnetic North Pole, with a difference of only an absurdly small degree (about 0.267). How could the Egyptians build their pyramids facing the magnetic pole? (By the way, compasses weren't invented until a few thousand years later.)

Rebuttal: I've done some research and you can use the sun. The ancients loved them some sun (and astronomy in general), and what they may have done was use shadows of sticks. It involves a few sunny days and a stick, so they'd have no problem, I believe.

Argument Three: There's a photograph of the Great Pyramid of Giza and its neighbour, seen from the Shinx, on the evening of the summer solstice. The sun sets in the exact centre of the two pyramids. For the Egyptians to be able to do this they'd have to know the exact date of the summer solstice and knowledge of the exact length of the year (about 365.25 days), a fact not discovered for a long time after. So the aliens came and told them the truth and built the pyramids.

Rebuttal- OH IT'S TIME TO TURN UP THE SASS.
(Of course, you know I'm not very good at turning up the sass, but that's another chapter entirely.)
As mentioned before, the Egyptians were awesome astronomers and they DID know, through observation of the night sky, the length of a year. (Yes, the exact length.) They had a legend accompanying it, something along the lines of a goddess wanting a kid but her hubby not wanting that so eventually Time Lord added one extra day every four years (yes, the leap year) so she could have her babies.
As for the calendar itself, it worked like:
Twelve months in a year.
Each month had thirty days.
Each week had ten days.
At the end of the year they'd have a few days left.
So long story short, ancient is not synonymous with primitive and Ancient Egyptians did know about the summer and winter solstice. (Also, it's just two days of the year where night and day are of the same length. Not that hard to figure out.) (Or was it longest day and shortest day? I think it was.)

Argument Four: A photograph taken on the day of the winter solstice from the entrance of the Great Pyramid. The big shape there is the sphinx. The sun traces around the sphinx's head while travelling along its path. It rises exactly on the left side of the base and sets on the right. They'd have to know the exact length of the year and stuff. Coincidence? Aliens!

Rebuttal: Astronomy. Don't underestimate the ancients. They made decent doctors, too.

Argument Five: The Belt of Orion lines up with the three pyramids of Giza, both in position and size. While possible, it'd create many difficulties for the Egyptians in terms of distance. Besides that, at the time they were supposedly built (3000 BC) the stars that made up the Belt weren't exactly at the correct angle to match up with the Pyramids. If the location of the stars is traced back, the time at which the belt is exactly aligned is around 10,500 BC. Also, if you look at the Sphinx, you'll notice the body is perfectly proportioned for the head of a lion, not human. The puny human head looks silly on its throne of lion. There are signs of water erosion all over it, and the last time there was any water nearby was around 10,000 BC and the constellation of Leo the Lion (closely related to the Sphinx) rose directly behind it in 10,500 BC.
Did the Egyptians build their pyramids in proportion to the stars but differently aligned them deliberately, and after they built Mr. Sphinx they made the head look small and funny and then pour water all over it to erode it? It was the aliens. They built everything in 10,500 BC. Thousands of years later Ramses decided he wanted a human head but the Egyptians weren't skilled in masonry so they built the head too small. Also, the Pyramids of Giza, built 3000 BC, are still standing and aside from the capstone are still perfect. The other pyramids, built around 500 years later, are all crumbling down.

Rebuttal: Because this is a long one, I'll make a list.
1. It's very possible to build the pyramids with a small margin of error location-wise.
2. The fact stands that lions and people are different. If the lion bit was weathered down and the human bit was added much later, it'd be less eroded and from what I can see, it isn't. It'd be pretty damn hard to make a lion-human look proportionate and normal. The lion bit represented strength and the human bit wisdom. In short, power.
… It actually looks like more of a monkey head to me.
3. Leo was a Roman (or Greek?) thing. I don't know how the Egyptians were with constellations, but if you look at the thing it'll take a very special person to see a lion.
Woah. Apparently this is invalid. A bunch of ancient civilisations saw a lion (Egyptians weren't mentioned).
4. Not sure about water erosion, so you got me there.
5. They were pretty skilled masons and an art can be lost between the generations.

Argument Six: If you take the permitter of the pyramid and divide it by twice its height you get pi accurate to the fifteenth digit. The chances of this happening by coincidence is remarkably small. Did they know what pi was? Not likely, seeing as it was only calculated accurately to the fourth digit in the sixth century.

Rebuttal: Pi is what you get when you divide a circle's circumference by its diameter. No matter the century, if you got a circle and decided to divide it by its diameter you'd get pi. It's not impossible the ancients didn't know this.
This'll probably reinforce the alienly-beliefs, but if you get the length of Giza's Great Pyramid's base from its centre, its height and the line joining them, you get the 3:4:5 ratio everyone's so fond of. Yes, Pythagoras: The ancient Egyptians knew about it. Not in the name Pythagoras, but they knew the rule nonetheless. The rules and patterns always exist; we find them.
Anyway, if you use the 3:4:5 ratio you'll see that it has a slope of 50 degrees and 52 minutes, which is remarkably close to the golden pyramid inclination of 50 degrees and 50 minutes.
Anyway, they probably didn't know they were doing this because they didn't have a sense of irrational numbers and the only right-angle triangle used was the aforementioned one.
But the golden ratio and Fibonacci are everywhere in nature and I wouldn't put it past the Egyptians to figure it out. This is getting long. I'll stop now.

Argument Seven: Even though the sides of the base of the pyramid is around 757 feet long (not sure how much in meters) it forms an almost perfect square. Every angle is exactly 90 degrees and the sides have a difference of around 2 cm, which is incredibly small.

Rebuttal: Maths. Not that hard. Squares are everywhere if you're interested in stable things. (Triangles are more stable, to be fair.)

Argument Eight: Egptians kept careful records of everything (leaders, wars, structures) but there were no records of the pyramids.

Rebuttal: They'd have been very careful to record the coming of aliens. Anyway, we only know what they did keep track of. What they didn't we have no idea about (as for wars, there were instances where someone on either side documented inaccurately).

Argument Nine: Egyptian didn't invent the wheel but the blocks carried would've weighed about 2 tons each.

Rebuttal: That's always a mystery, but they could've used rolling logs or something, and had groups to pull up the stone. If you use rubble as your ramp you can adjust it to make a very gentle slope to make the job easier.

Argument Ten: If you take the line of latitude and longitude the Great Pyramid lies on it's 31 degrees west and 31 north. (Coincidence?) They're the two lines that cover the most combined land area in the world. Therefore, the pyramid is the centre of all land mass on Earth.

Rebuttal: Coincidence that they cover the most land area on Earth, but doesn't necessarily mean it's the centre. Anywhere can be, when you shift around a little. As for the same number coincidence, yes, coincidence too. The numbers are relevant to Greenwich, London and the Equator and didn't come into effect until not too long ago.

Argument Eleven: A group of modern scientists attempted to build a pyramid next to thr real one using modern technologies (which pyramid not specified) and after around 100 days succeeded in building one around 1/40 of the real pyramid's size.

Rebuttal: A group of people and a hundred days isn't much when talking about the Real and Actual pyramids which worked something like a few thousand people over a few years. 100 days is less than a third of a year.

Argument Twelve: The height of the Great Pyramid is almost exactly a billionth of the distance away to the Earth from the sun.

Rebuttal: Not sure if they did this, but I wouldn't put it past them. I think you use trigonometry and Venus. … So yeah.

Argument Thirteen: All the fungi that was found in (quote) King Tutu's chamber. (I've heard a lot of nicknames, but that's my favourite by far. King Tutu.) Fungi that hasn't been seen before on Earth. The Pharaoh's Curse. (Quote) You decide, who built the Pyramids?

Rebuttal: Oh, God, we're almost done. Anyway. Not sure about the fungi, but there were traces of cocaine found in mummy corpses. Not sure why I said it, but fun fact for you. And new types of fungi can grow, right? (I've never heard of the fungi, actually.) As for the Pharaoh's Curse, you rob a grave and you deserve everything karma throws at you.