Now Celia gives the same push to the empty wagon and then to the loaded wagon.

The same push, 4 squares long, on the empty wagon and on the loaded wagon.

The empty wagon speeds up a lot.

The loaded wagon speeds up only a little.

The same push: a big change for the empty wagon, a small change for the loaded wagon.

The loaded wagon has more stuff in it.

How much stuff is in a thing is its mass, so the loaded wagon has more mass.

The same push made a smaller change in the motion of the wagon with more mass.

So, when Celia said the loaded wagon would speed up just as much, she was not right.

The loaded wagon speeds up only a little, because it has more mass.