Now you know

Why do aluminum beverage cans have concave bottoms? No, it's not a trick to make you think you're getting more than you are. The aluminum walls of a carbonated beverage can are only .005 in. thick - about as thick as a magazine cover. The walls can be so thin because the pressure of the carbon-dioxide gas stiffens the walls the same way a flabby balloon is stiffened when it's filled with air. That's also why the can bottom can't be flat: The pressure would push it out, and the can would rock on its bottom. The concave bottom acts like an arch dam to resist the pressure. The top can't be dished, though, so it has to be thicker - which is why the cans have a "stepped neck": a smaller-diameter top uses less metal.

Source: 'The Evolution of Useful Things,' by Henry Petroski (Henry A. Knopf, 1992)