Your heart is about the size of your fist.

Some trees can live for thousands of years.

Earth's inner core is about as hot as the surface of the Sun.

The first bicycle, made in the 1800s, had no pedals — riders pushed with their feet.

A day on Jupiter lasts only about 10 hours — the shortest day of any planet.

Your eyes can tell apart about a million different colors.

Earthquakes happen when tectonic plates suddenly slip past each other.