Check Your Air Pressure

Walking around my truck last night I thought the tires looked a little low. The tread on the tires is really good – I have only had these tires for a little over 2 years. So I drove up to the gas station and had to pay a dollar to use their air hose. I was able to get 3 of the 4 tires filled with air before the dollar ran out. The 4th one is not so bad that I wanted to spend another dollar on just that one. But I wonder why they had gotten so low on air in the first place?