Shift-Lock Design These cage cups with their shift lock design allow the cups to be slid down over chicken wire, and then slid over the vertical chicken wire, firmly locking them into place. This allows the cage to be moved to fresh grass, without the cage cups becoming unattached from the cage. The cage cups being locked into place also help keep dirt and vermin/insects out of your animals' water source. Fresh Clean Water = Healthy, Happy, Hydrated animals. Boil and Freeze Resistant These cage cups are BPA-free, boil resistant, and freeze resistant. Helping you to keep your animals' water sanitary, and allowing you to use these cups year-round. In the cold winter months, we recommend filling your cups 1/2 to 2/3 full, in order to make ice removal as painless as possible. If the ice gets frozen solidly into the cups, then just set them somewhere warm until just a little ice has melted, and then it should slip right out. Options to Fit Your Needs We have several sizes and two colors of shift-locking cage cups. In gray, we have: One cup, one pint, one quart, 1.2 quarts, two quarts, and three quarts. In black, we have: One cup, one pint, and one quart.