Because even a pre-schooler knows the difference between a circle and a sphere? I know that was pretty basic stuff to my then 3 and 4 year olds.
And as for your earlier example
it's quite definately a circle.
A ball is a sphere, a smiley face on a computer screen is a circle, as is a circle drawn in the sand with a stick.
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Asgara
"An unexamined life is not worth living" Socrates via Plato