Milk chugging, or the "gallon challenge," was a thing a few years ago. The rules are pretty simple: Drink a gallon of milk in 60 minutes, and keep it down. It is harder than it sounds, as many a high schooler and college student have discovered.

Popularized by the MTV show Jackass, "this activity hasn't proven deadly," says Miriam Weiner at U.S. News. But "gallon challengers inevitably face an unpleasant round of vomiting and possibly diarrhea, cramps, and bloating" after attempting the feat. (The Jackass video is here — I'm not embedding it because it is gross. You've been warned.)

Earlier this month, at Uncle Bob's Self Storage in Upper Saddle River, N.J., freakishly fast competitive eater Takeru Kobayashi not only successfully completed the gallon challenge — he obliterated it. (Watch above) Kobayashi downed an entire gallon of whole milk in 20 seconds. And that wasn't even the main event: He was merely washing down the 13 cupcakes he'd just swallowed in one minute — a feat Uncle Bob claims is a world record. Watch:

Kobayashi became a competitive-eating legend by winning every Nathan's Coney Island hot dog contest from 2001 to 2006, when he was frozen out from the contest over a contract dispute.

Mere mortals might want to think hard before drinking a gallon of any liquid in a hour, says Marshall Brain at HowStuffWorks. "Even a gallon of water can, potentially, be fatal if you drink it quickly." But milk poses its own challenges, Brain adds. It contains three "vomit-inducing" ingredients: Lactose, calcium, and casein.

The problem with lactose is that your body can only process so much of it at a time. You need the enzyme lactase to deal with lactose.... In drinking a full gallon of milk, most people burn through their available lactase and then become lactose intolerant during the challenge....

The problem with calcium is that milk contains a lot of it — about 300 mg per 8-ounce glass. A Tums chewable tablet contains about 200 mg of available calcium. So in drinking a gallon of milk, it is like downing 25 Tums tablets at once.... It messes up the acid balance in the stomach, giving your stomach another reason to consider puking. [HowStuffWorks]

What about casein? It "wants to react with the acid in your stomach and instantly turn to cheese," Brain explains. After a gallon of milk, "your stomach fills with stringy curds, which is yet another reason to puke."

The best way to win a milk-chugging contest was demonstrated by Indy 500 champion Dario Franchitti, who competed in a more-humane pint-of-milk-chugging contest on Late Night with Jimmy Fallon in 2009: