To hypothesize means to make an educated guess about why something happens or what might be true, based on what you already know. When scientists hypothesize, they use observations and knowledge to propose an explanation they can test.

Imagine you walk into your classroom and notice that all the plants by the window look healthier than the plants in the back corner. You might hypothesize that the plants near the window are doing better because they get more sunlight. That's your idea, your hypothesis, about what's causing the difference. Now you can test it by moving a plant from the dark corner to the window and watching what happens.

Scientists hypothesize all the time. Before conducting experiments, they hypothesize about what results they'll see and why. A biologist might hypothesize that a certain medicine will help patients recover faster. An astronomer might hypothesize that a distant star has planets orbiting it based on how its light flickers.

The noun form is hypothesis. When you make a hypothesis, you're taking that first step in solving a mystery: proposing a possible answer that you can then investigate. A good hypothesis is a thoughtful prediction you can actually test to see if you're right.