As Theodore Gray of PopSci explains, no matter how tough your lock is, there's one thing that will defeat it every time—and that's temperature.
Most bicycle locks are made of steel, which is incredibly tough, but it gets really brittle when it's cold enough; –13°F, to be specific. So how do you get a piece of steel that cold? With a can of compressed air.
Compressed "air" is actually difluoroethane, a chemical that acts as a refrigerant. When it's sprayed on steel, the tensile strength (how much force it takes to break) doesn't change, but the temperature change makes it less flexible. Less flexibility means it's less tough, so then, you can take a hammer to it and it will (eventually) crack. You can even shatter a really flexible rubber ball if it's cold enough.
So why aren't bicycle thieves using cans of compressed air to make off with every bike they see on the street? Because even though this trick can break the lock, it still takes quite a bit of hammering, which wouldn't exactly be subtle.
You can do this with almost any lock, though, so next time you lose the keys to your toolbox, this might come in handy.
Check out the PopSci article for more details and to see a video of this science "hack" in action.