An idea is a thought, plan, or mental picture that forms in your mind. When you have an idea for a story, you imagine how the characters might act and what could happen. When you get an idea about why your science experiment failed, you're forming a theory about what went wrong.

Ideas can be simple (“I have an idea: let's play basketball at recess”) or complex (Einstein's ideas about how gravity works). They can solve problems, create new things, or help you understand the world better. An inventor has an idea for a new machine. An architect has an idea for a building. You might get an idea for what to give your friend for their birthday.

Some ideas come suddenly, like a flash of insight when you're stuck on a puzzle. Others develop slowly as you think something through. Good ideas often come from combining things you already know in new ways, or from asking “what if?” questions.

The word can also mean a general sense of something. If you have no idea where you left your backpack, you're completely stumped. If you have a rough idea of how long something will take, you can make an educated guess even if you don't know exactly.