Why is this?
When you teach something, you have to understand it. You have to know it forward and backward. You have to do research. You have to practice. Once you're the expert, then you can present it to someone else.
So try teaching your little brother how to spell a new word.
Try teaching your mom how to do an algebra problem.
Teach your best friend about the scientific method.
Teach your dad about the battle at Gettysburg.
It's amazing what you'll learn in the process.