A protocol for two Italians to pass through a door

The protocol goes as below.

A: Please.
B: Please.
A: I insist.
B: So do I.
A: OK then, thank you.
B: You are most welcome.

The protocol is zero-knowledge because it can be simulated without knowing
any of the secrets of these Italians; in fact, the execution is independent of their
secrets as well as of anything else.

Loosely speaking, zero-knowledge proofs are proofs that yield nothing beyond
the validity of the assertion. That is, a verifier obtaining such a proof only
gains conviction in the validity of the assertion.

Source: Silvio Micali, 1985 and A Brief Introduction to Zero-Knowledge (by Oded Goldreich)