Sorry if my first post wasn't so clear. They're actually tires with a kevlar lining to resist protrusion (so less goatheads to pop your precious lil tube). If you want to eliminate having to ever change tubes, you can also throw down a couple more dollars and get thorn resistant tubes. Said and done you'll have some stiff tires and will tack on a couple pounds to the overall weight of your bike, but will never need to deal with flats.

Unless you ride through fields of glass, nails and mines.