Remember this picture of Earth from your high school geography textbook?

Crust > Mantle > Core Crust on the outside. Mantle in the middle. Core on the inside.

A coffee bean is a little bit like the Earth. As you go deeper into the bean, you find different flavors.

Acids and fats are at the crust. Sweet flavors are in the mantle. Bitterness is at the core.

Hot water reacts with coffee at the bean's surface, so the flavors closest to the surface come out first.

If your coffee is bitter, you've gone too deep!

Grind the beans more coarsely, or shorten your brew time.