People have been trying to discover the truth about dreams for centuries, and the simplest answer is this: dreams are basically stories and pictures our brains create when we're asleep.

Most dreams happen during the times of night when we are most deeply asleep, and our eyes begin to move around quickly under our eyelids. This may sound creepy, but it's totally normal, and it's called Rapid Eye Movement, or REM. Dream researchers used to think that REM was the only time people dream, but now most experts agree that we can dream at just about any time of the night. Maybe REM dreams are just our most memorable and realistic dreams.

The experts disagree on just why we dream. Here are some of their theories:

• Some say dreams don't really have a purpose; they're just one of those things that seem to happen for no reason.

• Some say dreams are our brains "twitching." Because our brains are basically huge collections of information, pictures, and feelings, when they "twitch" in the night, all kinds of strange things come out and get thrown into dreams.

• Some say dreams are a way to process all the events and emotions of the day, and are important to our mental and physical health. It's sort of like when you leave a computer on, it sometimes runs programs to clean up its hard drive. Our brains are always "on" even when we're asleep, so dreaming could be a time for them to do their own version of cleaning up the "hard drive."

• Other experts say that dreams exist to solve specific problems in our lives. Let's say you give a computer an incredibly complex math problem, and it has to take a few seconds before it comes up with the answer. This could be what dreaming is about, except instead of solving a big math problem, our sleeping brains are trying to solve emotional issues. The stories and images we experience as dreams are like way-out versions of our emotions, and our brains are working through those emotions.

"What are dreams?" is one big question, and in the next section, we look at another: What Do Dreams Mean?