Classic challenge for a woman to touch their elbows together, often at their navel. Large-breasted women may have trouble doing it, but either way, an attempt will result in the breasts being pressed together and possibly cleavage, much to the delight of the pervert that initiated the challenge.
The feat isn't much of a challenge: women not looking to please perverts will just touch their elbows together above the breasts, avoiding the obstacle altogether and having no effect. There will also be no effect if the breasts are too small, and the arms will just cover the chest anyway.
A harder, opposite, and equally perverted variant of this challenge is trying to touch the elbows behind the back (which most people can't do), causing the breasts to stick out.
