Monge: cards go alternately on top of and underneath a growing pile in the other hand. Milk: the top and bottom cards are stripped off together onto a new pile. Milk undoes Monge; each repeats after 10 rounds.
The deck · top first
Twenty cards in order. Monge them, milk them, and watch the pattern.
No shuffles yet.
Cycle lengths on 20 cards: Monge 10, Milk 10. Monge then Milk: back to the start in one round, because Milk is Monge's inverse.