There are two more naturals than sharps/flats; therefore, there must be two places where naturals touch. These naturally occurring half steps exist between B/C & E/F (no sharp or flat between). B goes directly to C, and E goes directly to F. [B# is C, Cb is B, E# is F, Fb is E].
The two pairs of naturally occuring half-steps are in the above. There isn't a sharp/flat between those tones (the two places on the piano with no black note between = B/C & E/F).