How to make the collision of two objects only change direction, don't change speed

again

How to make the collision of two objects only change direction, don't change speed, direction is normal physical collision direction of change。
Two spheres do uniform motion, only change direction after collide with each other, but the speed is constant.
If direct collision with "change velocity", you must know the object after the direction of the rebound.
With trigonometric function formula to calculate collision after may first movement direction.
Again "change velocity" after collision Trajectory, speed.
I don't know this method line not line,
I Want to ask a solution.
(because I don't understand English, so this text is to use translation software, there will be A lot of grammar mistakes, want to use your imagination to coherent)