Talk about a dirty trick! Bill Gates pulled one over on Jimmy Fallon on The Tonight Show on Wednesday, Jan. 21, when he stopped by to chat about his investment in the Omniprocessor and then tricked the host into guzzling a glass of "poop water."

The Omniprocessor, as Gates explained, inexpensively turns sewage into drinkable water. "It's way too expensive the way that the rich countries do it," he told Fallon. "[So we challenged engineers to] make something that we can put throughout Africa and get rid of all that sewage."

Fallon then brought out two numbered glasses of water, one of which, he said, contained poop water. "This is the ultimate taste test," Gates said. "We think you'll be able to figure this out."

"Already I've got a guess," the Saturday Night Live alum quipped, fiddling with the "No. 2" glass.

Gates offered to let the comedian choose which glass he wanted to drink, but Fallon said he'd take whatever the Microsoft co-founder gave him.

Both men chugged their respective glasses of water — after which Gates revealed there was a catch in the game.

Watch Fallon's funny reaction above!