Here, a number of steel balls are suspended serially with the help of a hinge at the top. Hold a ball in hand and allow it to strike another ball. Try the same thing by holding 2 steel balls together and allowing to strike the remaining idle balls. Repeat with 3 steel balls and then with 4 steel balls.

What did you observe? 
Even though the number of balls striking change, the remaining balls do not move.

The science behind it :
The remaining balls do not move due to their resistance to the movement caused by the inertia.