Title says it all. Every balloon that I have got, from parties, etc, all slowly deflated for no reason at all.
Why is that?
Balloons are not perfectly air-tight. Air molecules can slowly diffuse through the surface, or leak through the knot. Of course, they can diffuse in just as well as out. The reason there is a net flux directed outward is that the pressure is higher on the inside, since the air there has the force of the wanting-to-shrink balloon to support in addition to atmospheric pressure.
You'll probably notice this effect proceeds faster with helium balloons. This is a result of helium existing as single, almost perfectly non-interacting atoms, rather than as molecules with two or three atoms. It simply has an easier time slipping between the cracks and diffusing through membranes.
One final note: The pressure imbalance is not particularly large. To get a feel for this, remember that atmospheric pressure is 14.7 pounds per square inch, so a balloon with a 5 inch radius has well over a thousand pounds of force being exerted on its surface by the exterior air. Since you can inflate a balloon with just your lungs, you know the total force induced by the stretched membrane is not terribly much. Thus you'd expect that the total number of air particles entering and leaving the balloon far exceeds the net outward flux.