Cooling fans.

Why do all cooling fans have a prime number of fans? I see 3 blades, 7, and 11 right in front of me. What is the reason for this?

Wow..I have never even realized that. Add in a few 5 blade fans.

It's so that if one of the fan blades breaks off, you can't rebalance the fan by breaking the fin on the other side off. Thus making you buy a new fan. It's a really complex marketing scheme.

I think that it has something to do with fan balancing. And it's not really prime but odd, as I've seen 9-blades too. If you apply force to odd number of points, middle is more stable, like desk chairs with wheels - have usually 5 "legs". This way force applied to 1 point (blade) is equaled by at least 2 points (blades).