 concrete, and that can cause cracks. Also, the soil underneath the foundation or beneath column supports may have shrunk due to moisture loss, or if you have too much moisture around the house, the soil under the foundation can weaken. And some soils also are expansive and very sensitive to the amount of moisture. So lots of reasons. The idea is, I mean, what you really have to do is keep your eyes out in the formation of cracks and track them over time. It's one of the ways you know how serious it is.