Besides the obvious (wings), birds and insects are able to fly because of their bone density, which is little to none. So yes we're too heavy.
Evolution is closely related to the environment a species lives in. It's possible that bats and birds evolved to fly because they lived in forested or mountainous areas where flying is more efficient than climbing, and probably needed to fly to escape predators and find food. Humans specifically evolved in the grass-plains of Africa, where flight wasn't necessary for survival. Other mammals evolved either in jungles where there is plenty to climb, or oceans.
... I have a lot of questions like that too, it's why I'm studying biology. :P
I'm pretty sure Rori knows how/why bats evolved to fly, but she was curious why they were the only mammals that did. For instance, other forest-dwellers or mountain-dwellers or what have you. The point of the strip, though, was that husband didn't have any answers...he's kind of dim, after all.
Ah, ok. I got the joke. I just can't leave a question unanswered. I'm serious, right know it's just my hands typing this against my will. My will just wants some pizza.
Well, I guess I should have explained better. Yes there are other mammals in forests and mountains, which probably DROVE bats to fly so they didn't have to compete for food and such. Mountain goats and sheep are perfectly happy hopping around eating grass, so much so that bats needed to find caves high in the mountains for food.
And while flying squirrels don't truly fly, I think in a couple million years they'll have mastered it.
I hope I explained it a bit better, I looked at that wiki article and it didn't seem to work. And again, AGAINST MY WILL!!!