The 4-to-1 MUX

The last lesson built a 2-to-1 MUX: one select line s chose between two inputs. One line has two states, so it can name two inputs.

To choose among four inputs you need to name four things, and one bit cannot. Two bits can: $s_1 s_0$ counts $00, 01, 10, 11$, which is exactly four addresses.