If you've ever stopped to put air in your tires, you'll know the air is inserted into the tire through the tyre's valve stem. The valve step is protected by a small plastic cap screwed onto the stem.
Losing a Cover
A tyre's valve cap can come loose and fall off your tire for any number of reasons. If it's not tightened enough, the bumps of the road can cause it to loosen over time. If it's tightened too much, the thread on the valve stem may become stripped, causing the cap to loosen. It can also be loosened or knocked off if you brush against a curb while parking.
There is generally no negative immediate consequence associated with a valve cap falling off, but there can be some long-term problems. When the cap is removed, the valve stem is exposed and can get clogged with road dirt, grease or ice. This can make inflating the tire difficult. A missing valve cap can also lead to a slow air leak over time, eventually resulting in a tire going flat.
What to Do
If you find your valve caps are often loose or missing, you can buy locking valve caps at any automotive store. This style of cap actually locks onto the valve stem and is only removed through a special key that is sold with the cap. This type of cap is also helpful to prevent vandals from letting the air out of your tires.