Wally was relying in Newton's third law of motion, which is commonly rendered as,
"Every action has an equal and opposite reaction." That is, all forces result from the interaction
between two objects, and when two objects interact, they apply an equal amount of force to each
other, with the forces acting in opposite directions.
The difference that we see in movement is due to Newton's second law of motion (a=F/m,
commonly rendered as F=ma), which shows that object with a lot of mass does not accelerate as
much. Similarly, when you bounce a tennis ball off a brick wall, the wall does accelerate, but if