@Al: Over time they dry out. Good prctice is not to stoop over it when applying voltage to them! If they are large, it may be worse than the need for new underwear!
Don't underestimate the force when they go off, the gunk coming out the open end is nothing compared to the can shooting off in the opposite direction. I know, many years ago a ham friend of mine said on the air that he heard a strange sound in his PA. Then he went off air. Later he told me he put his ear down to the PA, and luckily there was a lid on it. He did this just as it went off, and got a bit scared to put it mildly! I was shown the mess, and can understand his reaction.
