Why does the newton's cradle stop swinging

The balls are not perfectly elastic, but the main reason is probably air.
Explanation:
When 2 adjacent balls have any distance between them, there is air in the space. When the moving neighbor comes at a stationary ball the moving ball has to push the air out of the way.
I hope this helps

The independent variable is the type of music.the dependent variable is the pulse rate.the intervening variable is the age of the subject.if the mother does not like rock, her pulse is likely to not increase.the same might be true for the sister.the friend may like rap and gets excited.to establish cause/effect relationship, all of the subjects must be consistent.hope i .