The shells of all the eggs I use go back to my girls. While eggs are frying in the pan, shells are baking in the oven. I have a metal bowl that I toss them in. Pop it in the oven as it preheats. When it beeps, I turn off the oven and just leave them in there. After they are cool, I crush them and toss in the next eggs I use and continue the process. When I get a bunch of treats we mix them in the same bowl and toss them in the run.