Imagine rafting with a group of friends along a winding river on a hot summer day. At noon, the group decides to take a lunch break. Everyone jumps out and helps to pull the raft on shore. Its very hot, so your friends find a shady spot a few yards away to eat lunch. Later, when you return to the raft, you are dismayed to find that the raft burst while sitting in the sunlight. What caused the raft to burst?