A man is out walking his dog beside a lake when he suddenly sees a woman just managing to keep her head above water, but then slowly sink. He dives in, grabs the woman and pulls her to the edge of the lake. He places her on her back, raises her arms and starts making pumping movements. Each time he pumps, a thick jet of water shoots out of her mouth.
In the meantime, a cyclist has stopped and is watching the events, shaking his head. The man keeps pumping, but each time a thick jet of water still shoots out of her mouth. The cyclist just shakes his head and says, "That's never going to work."
"Shut up! I know what I'm doing, I'm a doctor."
"Well," says the cyclist, "I'm an engineer and I'm telling you if you don't take her arse out of the water you'll pump the fucking lake dry."