Have you ever noticed your riding buddy "scooting" down the trail on his ass? It might look something like this. Not a pretty sight, and a habit I see on the ski slopes too. There is a scientific reason for "scooting" and although it has traditionally been a canine affliction, it now seems to be making its presence known in humans with an active lifestyle, as well.
Here is an excellent site that explains this phenomenon and has an interactive, virtual practice area where you can try your hand at treating the "scoots". You'll definitely want to install the Shockwave player for this one. It's worthy of a hearty Friday chuckle...until your buddy asks if you can help him out with something.