Wow that's not a broad and slightly retarded statement. Running is not bad for your knees if done correctly and sensibly.
Biking is bad for your knees? Are you kidding me? Apart from swimming cycling is one of the number one things recommended to people rehabilitating their knees who cannot run. It's bad for your knees? How? It's no impact, and unless you ride you bike like a clown on a minature tricycle, it's not going to be damaging for your knees.
Not taking anything away from swimming however, but I did want to point out the inaccuracies in this post.