If you have not noticed yet, you will find that the F and J keys have a small bump on them. The bumps on the F and J key are also called nipples or ridges.

Ever wondered why are they there?

These bumps let you place your index finger on these keys; the right index finger on the J key and the left index finger on the F key without looking at them. This helps you in placing all your fingers on the keyboard in an optimal manner.