Size matters, too. Doreen only models children's toys (most of those hands in ads you see during your My Little Pony marathons belong to grown women) because her hands are sufficiently dainty. "If I were to model an engagement ring, my hand would look childlike. I don't know how many viewers would pay attention to that, but, when you're making a commercial, they pay a lot of attention."
Adele only became a parts model because, at 5 feet 7 inches, she was considered too short to be a regular model -- and really, what's the point of being a model unless you can dunk on the photographer? Though her decision was arguably more lucrative -- hand models have much longer careers -- the early audition process was weird. "I went to a casting for [a nail polish ad]. I sensed a different reaction than what I would usually get at casting ... people were called from cubicles, and, suddenly, the whole board room was staring at my hands and passing me around."
Actually learning how to use the nail polish came later.
A typical hand audition involves lots of pictures and pretending to, say, spoon yogurt without suddenly overthinking every aspect of that process. Doreen, meanwhile, had to beat out actual kids so she could get a job pretending to be a kid, because actual kids usually don't have the patience to do long shoots or have the bills to stay motivated:
"They had a little desk. There was a doll and this little toy dog. The director wasn't in the room, so I just hear this voice floating over Skype saying, 'OK, Doreen, I want you to grab the dog and have it walk over to the doll and jump on her shoulder.' So, I do that, and he says, 'Great! Now, I want it cuter!' So, you bounce it a little more on the table. You're making it bounce back and forth so it looks like it's walking all cute."
Only those with seniority learn how to make them look like they're pooping.