Joiner on a 3rd floor of a building site forgot to bring his saw up with him. Shout down, but his co-worker can't hear him. Points to eye (I), then his knee (need) and moves his hand back and forth in a sawing motion.
Mate nods, pulls down his pants and starts to wank.
The furious joiner runs down the stairs and says "What the hell are you doing? I said I need my saw!!"
His mate replies "I know, I'm just letting you know I was coming."