The physics apparatus / executive toy known as Newton's Cradle demonstrates conservation of energy and momentum through elastic collisions. The NewtonsCradle.xaml file demonstrates SplineDoubleKeyFrame animation. The balls slow down at the apex and then pick up speed again.
The XAML version is silent, alas. I tried to add sound effects using a MediaTimeline element, but it kept crashing. Maybe it's time for new CTP. (If we should all start chanting "New CTP! New CTP!" our voices will be heard even as far away as Redmond.)