Mason sand. It's just like playground sand. There are other kinds and I would not recommend some of the others just because there are different sized pebbles and some can be sharp, and the wicked lil chickens would end up getting one caught goofy and have a whole line of issues. Did not know that chickens are attracted to and attempt to eat styrofoam. Just my own lil side note. Concrete sand may work better if it is going to get wet and be outside. Inside definitely use mason sand. Concrete sand outside. The extra lil bits in it will allow airspace and it won't cake up as bad and for a crust.