Wood expands and contracts with moisture content. More moisture makes the fibers "fatten up".
The interesting thing is that this is anisotropic: the expansion/contraction occurs across the grain, NOT along the grain. The rate of expansion also depends on the local characteristics of the grain itself (hence the effects of warping due to uneven expansion) ... Also there's a big difference between the direction "across the growth rings" (i.e. radially when it was still a tree) and tangentially to the growth rings. And these surfaces are curved, of course. But one thing we can always say is: the wood doesn't significantly change size along the grain.
Design and construction methods can make wooden artifacts more or less susceptible to cracking and distortion from this. For example dovetail joints can be pretty good as all the wood expands/contacts together the same way. Especially if the pieces are joined together from the same piece of wood. Stuff like that. Or at the other extreme, metal fixings like nails don't move with moisture at all, which can cause problems with relative movement and stress can accumulate.
Edit: and the repeated cycling of moisture content induced stress can eventually lead to cracking, in a similar way to metal fatigue. Old wood just cracks sometimes, this is probably why.
The interesting thing is that this is anisotropic: the expansion/contraction occurs across the grain, NOT along the grain. The rate of expansion also depends on the local characteristics of the grain itself (hence the effects of warping due to uneven expansion) ... Also there's a big difference between the direction "across the growth rings" (i.e. radially when it was still a tree) and tangentially to the growth rings. And these surfaces are curved, of course. But one thing we can always say is: the wood doesn't significantly change size along the grain.
Design and construction methods can make wooden artifacts more or less susceptible to cracking and distortion from this. For example dovetail joints can be pretty good as all the wood expands/contacts together the same way. Especially if the pieces are joined together from the same piece of wood. Stuff like that. Or at the other extreme, metal fixings like nails don't move with moisture at all, which can cause problems with relative movement and stress can accumulate.
Edit: and the repeated cycling of moisture content induced stress can eventually lead to cracking, in a similar way to metal fatigue. Old wood just cracks sometimes, this is probably why.