There are different types of collisions. Sometimes energy is lost. Sometimes the objects stick together. yet, in all collisions, momentum is conserved.

Elastic

Just a the word “elastic” implies, in elastic collisions objects collide, stay separate (ounce off), and all of the kinetic energy is conserved.

Inelastic

In inelastic collisions the two objects collide, stay separate, but kinetic energy is not conserved.

Where does the energy go? Its turned into sound ( a crash), damaged objects, or heat.

Newton’s cradle is an obvious example of an elastic collision. You can tell the kinetic energy is conserved because both the left and right ball rise to the same height.