Free
Try these riddles: How are a baseball, a flower, a brick, and a nail alike? How is water like oil and vinegar? How is air like helium? How is what’s in a fluorescent light bulb like lightning?
Here are the answers: Everything in the first group is solid, the second group is liquids, the third group is gases, and the fourth group is plasmas. Solids, liquids, gases, and plasmas—these are the four states of matter. Everything within our knowledge is one or another of them.
