A red hand stencil. A series of lines that look like a ladder. A collection of red dots. These images, painted in ocher on cave walls in Spain, are the oldest-known examples of cave art ever found. And all three were created not by humans, but by Neanderthals, scientists say.
Most of us think of time as a way to measure things like the length of our days and the span of our lives. But if you had access to a pair of extremely high-precision clocks, you could use time in a different way — to measure the height of mountains.