Now at some point in my youth, I earned some money of my own and spent it at the pet store on some mice. Soon, it was more mice, and then, even more mice, and I got to see mice killing and eating other young mice. At this point I learned how to sex mice, having already learned why one should sex mice. You need two cages or container to sort them into, and you have to keep track of which container is which. Separating the males from the females only works if you get all of the males in one cage and all of the females in the other: no exceptions. One female in with the males means more mice. One male in with the females means a lot more mice– a whole lot more mice.