What's the Sign for "Sign"?

Here is the Monkey's idiosyncratic sign language vocabulary:

ANIMALS
dog
cat
horse
elephant
pig
tiger
bird
owl
primates generally (monkey, ape, gorilla, orangutan, etc.)

VEHICLES
helicopter
airplane
steamshovel/backhoe

ACTIONS
eat
drink
let's go that way! (just kidding: it's a point)
nurse/milk
goodbye

MISCELLANEOUS
guitar
ceiling fan
flower
church (we showed him once, he picked it up the next day)

I am probably missing a few. I'm glad we started doing the BabySign thing. He gets so much satisfaction from signing a word, or hearing us say a word he's just learned, or just the fact that we "get" him.