A hypothesis is an educated guess about how something works or why something happens, one that you can test to see if you're right. When scientists notice something puzzling, they form a hypothesis before they start experimenting. It's based on what they already know and can be supported or disproven through observation or experiments.

For example, if you notice that your mom's tomato plants grew taller this summer than last summer, you might form a hypothesis: “I think the plants grew taller because we had more rain this year.” Now you have something you can test by measuring rainfall and plant growth.

Your hypothesis is the foundation for your investigation. In science class, you typically form a hypothesis before conducting an experiment. A good hypothesis is specific and testable. “Plants need water” is too vague, but “tomato plants that receive one inch of water per week will grow taller than plants that receive half an inch” is a proper hypothesis.

A hypothesis is where scientific investigation begins. After much testing and evidence, a hypothesis that holds up consistently can contribute to a scientific theory.