Ready for a physics lesson? When something bounces, it's subject to the pull of gravity as it rises, which is why it stops at a certain point--the peak of the arc--and then falls down to earth as gravity pulls on it. As it rises, gravity is going to pull harder on its weight over time, so as it approaches that peak it's going to slow down--but after it crosses the peak and starts to fall downwards again, the pull of gravity will have the opposite effect and cause it to move faster and faster as it descends. So our animation needs to do the same thing; it needs to start off fast as it leaves the ground, slow down as it approaches the peak until it stops, and then pick up speed again as it hurtles back to earth.
So first we'll handle the slowdown on the first rise of the snowball. Click on your timeline in the range of your first motion tween, the one that bounces the snowball into the picture. You can click on any of the frames between the first keyframe and second, spanning the distance between the ground and the peak of the arc. Then, in the Properties pane, click on the slider for the Ease control, and drag it upwards until it increases, so that the animation speed eases out.
