One region of the brain that contains a lot of THC receptors is the hippocampus, which usually helps with memory. When THC attaches to receptors in the hippocampus, it interferes with memory.
Researchers have also shown that heavy use of marijuana by young people can actually cause IQ to go down—and this change in IQ can last a long time and may even be permanent! This means that someone who uses marijuana may not do as well in school and may have trouble remembering things like their friend's phone number.
Maybe you've heard that marijuana can cause a range of emotions from uncontrollable laughter to paranoia. That's because THC also influences emotions, probably by acting on a region of the brain called the limbic system.
And don't forget this: THC can make something as simple as driving a car really dangerous.