One day, two off-duty paramedics are out walking by their local canal when, a few hundred yards along the bank, they see a guy push another into the water and run off. They race up, but the assailant has long since disappeared and the unfortunate victim is now floating face-down.

One of them dives in and drags him to the bank; they heave his torso out of the water and begin to apply artificial resuscitation by pumping on his rib-cage. Pump, pump... lots of green water comes up... pump, pump... green water, fish, Coke can... pump, pump... green water, old saucepan, frog...

"Boy," says one rescuer to the other, "has he swallowed plenty! Looks like this could take a while." Pump, pump... green water, bike frame, green golf ball :-), bedstead...

While all this is going on, another guy arrives on the scene. Taking in the whole situation at a glance, he hurries over and asks, "What's going on here? What are you doing to that poor guy?"

"Don't worry," says one of the rescuers. "We're qualified paramedics; we know what we're doing".

"That's as may be," says the new guy, "but I'm a qualified engineer and I'm telling you, if you don't get his backside out of the water, you're going to pump the canal dry!"