Re: How come a balloon shrinks in cold air?

Most things expand when they are warmed and contract when they are cooled.
This is true for air as well.

The air from your lungs is relatively warm.  When you take the balloon
filled with this warm air out into the cold, the air inside the balloon
will be cooled as well.  This means that the air molecules inside the
balloon have less energy; they will move more slowly, and will not strike
the inside of the balloon as hard as they move about inside.  This 
results in a decrease in pressure inside the balloon, which makes it 
shrink.

To test this, if you take the balloon back inside a warm house, it should
expand again.  And if you hold it over a pot of boiling water, it should
expand even further. 

A more striking demonstration can be performed by heating a plastic milk
or soda bottle in boiling water for a minute or so, then capping it 
tightly and pouring ice-cold water on the outside.  The pressure inside
will drop rapidly as the air inside is cooled; as the pressure outside
(atmospheric pressure) has not changed, it will continue to press down
on the outside of the bottle until it crumples.