Physics
Conservation of Angular Momentum
Quick fact
A neutron star, the collapsed core of a supernova, spins hundreds of times per second because its angular momentum is conserved as it shrinks from a star thousands of kilometers wide to just 20 km across.
Why this is interesting
Have you ever wondered why an ice skater spins faster when she pulls her arms in, or why a spinning top doesn't fall over right away? The answer lies in a hidden force that refuses to let go.