What is 1 + 1?

That was a very short break. The gates have another job: addition.

Give them two one-bit numbers, A and B. They can arrive in four different pairs:

0 + 0 = 0

0 + 1 = 1

1 + 0 = 1

1 + 1 = 2

The last sum does not fit. One bulb can show 0 or 1. It cannot show 2.

The answer needs more room.