Farting vs pumping

At dinner one day towards the end of last year, we noticed a strange smell.

"Alex," I said, wrinkling my nose, "did you just fart?" (If you don't have kids of your own, this will sound strange, but it's true: parents can distinguish the smell of their kids' farts. Also, we can usually tell by the smell of their breath and body odour when they're about to fall ill.)

"No," Alex said.

"Well it certainly smells like you farted."

"I didn't fart...I pumped."

"You did what?"

"I pumped. A fart goes like, 'brrrp', and a pump goes like, 'pffft'."

"Ohhhkay."

I'd never heard of the distinction before, but since then, we've had this definition confirmed by a number of sources around Edinburgh. I guess it must be a local thing...