... because you will both have to SPIT at the same time (because preschoolers TOTALLY ROCK! at copying you!) and you will undoubtedly get an EAR FULL (literally) of berrybubblesomething flavored toothpaste laced dragonbreath spittle.
Because, while excellent mimics, preschoolers are lacking in the AIM department.
(This is especially true for boys as evidenced by the floor, walls and anything else within a 3ft proximity of my toilet.)