Why do you sometimes feel like you're jumping off a cliff in your dream and then jolt awake?

because the stuff your mind generates is actually sort of like a virtual world. sometimes when your mind dreams stuff it affects what happens in your REAL life, which is why people have wet dreams. You jump because if you dream your falling off a cliff, your brain senses it and wakes you up, because if you hit the ground, your mind actually senses that and damage could come to you.