How fast

Mikey, and now Roger, is really obsessed about "how fast" things are.
"Mom, how fast is a chicken?"
"Mom, what's faster, a rabbit or a dog?"
"What's faster, a giraffe or a t-rex?"
The boys will swing their arms in a circle "so so fast", then because they are twirling their arm so fast, they'll ask me if I can see their arm. Then they will spin in a circle and ask if I can see them then!?!!
I usually tell them that I can't see them or their arms in these circumstances, but then when they ask me how fast their arm is spinning, I really don't know what to say.
While in Utah, the boys asked LeNae this question, to which she replied, "It was a 7 out of 10"
Mikey then asked, "How fast is that?"