Ok, this isn't really photography related....wait! Yes, I guess it can be, most of us use cars to get to locations to photograph, so maybe this does have some relevance after all. Why is it that we always have to add air to the tires? Have you ever taken any out? No, but about once every few months, you have to put more in! Where does it go? I thought that tires are supposed to hold air? No, I don't have any leaks, punctures or bad valves. I've noticed this with every vehicle I've owned and every set of tires. Soak them in a bucket (ok, a big bucket!) of water, you don't see any leaks.....so then, where does that air pressure go??????