Question

what makes some collisions elastic and others inelastic?
if there is energy lost in the collision to sound, heat, etc., the collision is inelastic.
if there is no loss of kinetic energy in the collision, it is inelastic.
if the collision involves bouncing, it is elastic.
if the collision involves sticking together, it is elastic.

Explanation:

In elastic collisions, kinetic energy is conserved. In inelastic collisions, kinetic energy is not conserved as some energy is lost (e.g., to sound, heat). Bouncing doesn't solely define an elastic collision (as some energy could still be lost in a bouncing collision in real - world scenarios), and sticking together is a characteristic of a perfectly inelastic collision (a type of inelastic collision). If there is no loss of kinetic energy, the collision is elastic, not inelastic.

Answer:

If there is energy lost in the collision to sound, heat, etc., the collision is inelastic.