Science
Why the world looks and behaves the way it does.
1:40How natural selection works
Natural selection in four steps: variation is inherited, more are born than survive, the fittest pass the filter, and deep time draws the tree of life.
1:56Why moving clocks run slow
A clock made of light, sent past at 0.6 times light speed, ticks 4 times while ours ticks 5. GPS satellite clocks lose 7 µs a day this way.
2:14How a black hole forms
A heavy star collapses when its fuel runs out. Squeeze the Sun to 6 km across and escaping it would take more than the speed of light.
1:52Why the Moon has phases
The Sun always lights half the Moon; its phases are the angle we see that half from, not Earth's shadow. One lap takes 27.3 days, a cycle 29.5.
1:40Where rain comes from
A raindrop is about 100 times wider than a cloud droplet, so it holds a million times the water. Drops join as they fall until they are heavy enough to rain.
1:46Why is the sky blue?
Air knocks blue light aside about 6 times as often as red, so blue reaches you from all over the sky. At sunset the blue is gone before it arrives.