Billy was walking along a beach, which he often came to with his family. He would swim and have fun there. He, like many young boys, also liked to jump on sandcastles. And today Billy came across a particularly large and intricate sandcastle...

Alex liked building in the sand. He would spend hours on the beach, digging and piling up sand. The sea would destroy his work, but he accepted this as necessary. What he didn't like was people damaging his castles. So, while making a particularly fine sandcastle, he gathered some sharp stones and placed them pointing upwards...