GorillaJTA
That guy.
Make sure you check each board you pick for cupping (when the middle section of it sags but the ends are at the same height) or warping (when one end of the board is twisted). To do this, take the board off the rack and put one short edge down on the floor, wide side up. It should look like a sliding board-- nice and smooth and flat-- if you look down the length of the board. Then, turn it so the 1" edge is on the floor and look down the length of it again. It should be straight that way too. If it's not, set it aside and get another. Warped wood is hard to work with, and a cupped shelf will collect pee and poops in the middle and get really messy.
Haha, I have a shelf made from a bowed (cupped?) piece of wood and I flipped it upside down and now it is a little hill. At least it does not collect poop as the poop rolls right off!