Imagine two bodies A & B. they have same mass and same speed, but they are travelling in different direction. the kinetic energies of both the bodies is equal (1/2*mv^2), because they have same speed and mass. now they collide each other. As they have same mass and speed they both stop. now the kinetic energy of both the bodies is zero. but where is the kinetic energy gone......in other words what is it transformed into. but what about law of conservation of energy??