See and hear the smallest neighboring-key movement on the piano, then combine two half steps into a whole step.
Count every key
Move to the closest key
From any piano key, move to the very next key to the right or left, whether it is black or white. That is a half step.
Find the white-key neighbors
E to F and B to C are half steps even though both keys are white. There is no black key between them.
Combine two half steps
From C, move to C-sharp, then D. C to D is one whole step because it spans two half steps.
Counting only white keys can hide the real distance. A whole step may connect two white keys, two black keys, or a black and a white key. Count neighboring keys to check the interval.