“A father wanted to reward one of his three children with lots of gold! So, he gave them a test. He gave them a small amount of money and told them to buy something that could fill up an entire room. Susan, the eldest, bought lots of straw, but it wasn’t enough to fill up the room. John, the middle child, bought water, but he was still not able to fill the room. Peter, the youngest, bought two items and he successfully filled the entire room with something. The father gave him the gold. What two things did Peter buy?”
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Brett Williams,
Riddles for Kids: 150 Riddles and Brain Teasers That Will Leave Kids and Their Families Stumped