How does erosion form canyons?

It begins at a low area with a slope where the water runs down hill and VEEEEEEEEEEEERRRRRRY slowly washes away the dirt and rocks from the sides. So, something like the Grand Canyon took millions of years to form.

Very slowly. The Grand Canyon goes almost as far back as Adam and Eve, five thousand years ago.

Canyons ARE erosion. Go take a hose with a spray nozzle and blast some dirt with it. Don't let your brain get lazy! Air and water have the same effect on dirt and stone. Think!

Mechanical and chemical friction.