Q: We have had a large dining table returned to us this winter after four years of use. It had a glass top. What happened is that the table crowned - the top warped with the center being higher than the edges. The glass cracked too. Even though it is four years old, we will probably replace it in the interest of good "public relations." Can you tell us what has happened?

A: It is often hard to reconstruct the past events that cause wood to perform poorly in use. However, as I have stated in this column many times, wood does not change its size or shape unless its moisture content changes. With the crown, we know we have had a recent moisture change. We have two options: 1) The bottom side was exposed to extremely dry conditions while the top was kept more humid. (How could this be? Any dry air on the bottom would also be on the top too, wouldn't it?) Or, 2) The top was exposed to wet or very humid conditions while the bottom was drier. I vote for this second option.

I can imagine that someone could spill water on the top, it would sneak under the glass top and eventually soak into the wood. Maybe there was a plant on the table and the pot had a small leak that resulted in wetting over a period of time. This moisture would cause the top to expand, while the bottom, exposed to normal dry air in the house, stays the same moisture content and therefore the same size. The hardware on the bottom would also help hold the bottom from changing size. You might say, "This wet flower pot idea doesn't hold water!" Maybe it was a leaky aquarium then. But, I think we are dealing with a very rare event (the table crowning) and so have to look for a strange moisture event that would cause it. How else could we get the top quite wet for a period of time without setting the bottom? Perhaps the table owner knows, but it will be difficult for us to find out the truth. Can you see any water stains or rusty fixtures or fasteners?

In any case, you are being very generous in replacing the table after four years of trouble-free service.