I don't know. I think it began when they were little (I have had all 4 of mine since they were babies) and I put untreated wood chips in the cage and then in a corner I add some cardboard fluff stuff and they eat it in the beginning and then they pee on it. My chillas pee and then cover it and pee and cover it and it is adorable. I just have some good babies. Mine come right to the cage with their names, or to me. That happened from container jingling and saying their names. I also take them out and they sit on my lap and my boyfriend and i sort of just continually say their names...! Now if you say Hay...she things you are calling her and comes to the part of the cage closest to you.