What's a "snakebite"?

Snakebite flats are usually caused by the tire and tube being pinched between the road and the rim, causing two small holes in the tube that look like a snakebite. The usual causes are underinflation, too narrow a tire for your weight, or hitting something (rock, pothole) while having your full weight on the tire.

The obvious solutions are to make sure your tires are inflated properly, use a larger size tire if you weigh a lot, and either avoid rocks and potholes or stand up with your knees and elbows flexed (to act like shock absorbers) when you go over them.