Most tires today do not have a tube inside of them. There are grooves in the wheel that let the tire be popped into place and hold a lot of air pressure. If there is a crack in the tire, it will not be able to hold its air. Tire leaks are very common. The most common cause is a hole from a nail or screw. This can usually be fixed by patching the inside of the tire so that it can hold air again. If the hole is close to or in the side of the tire, it can not be fixed. This is because the sides of the tire flexes to support the weight of the vehicle. A patch will not be able to handle the stress.