A 0.25 kg ball moving at 2.2 m/s hits an identical ball at rest, head-on and elastically.

Step 1. (a) Equal masses. $v_1 = 0$ and $v_2 = u = 2.2\ \text{m/s}$: they swap velocities.

Step 2. Light hits heavy. $v_1 \approx -u$ and $v_2 \approx 0$: the light ball bounces straight back, like a ball off a wall.

Step 3. Heavy hits light. $v_1 \approx u$ and $v_2 \approx 2u$: the heavy ball barely slows, and the light one flies off at twice its speed.