A boulder is a really, really big rock. You can't pick up a boulder, but you can sit on it. The sport of bouldering came from climbing on boulders. A cement block is heavier than a brick. A mountain is definitely much bigger than a hill. A mountain is definitely way bigger than a hill. If you wanted to climb a mountain, you would need special equipment. A lake is a large body of fresh water. A lake doesn't have salt in it. A pond is a very small body of fresh water.