Writing with ancient bones

On the first day of class, it's not unusual for a teacher to put chalk to blackboard to spell out her name. What she's really doing, as NPR science writer Robert Krulwich put it, is "crushing the skeletons of terribly ancient earthlings" against a hard surface to visibly communicate her identity. Behold what a piece of chalk looks like under the microscope:

Krulwich explains:

"Chalk is composed of extremely small white globules. They look, up close, like snowballs made from brittle paper plates. Those plates, it turns out, are part of ancient skeletons [single-celled phytoplankton algae] that once belonged to roundish little critters that lived and floated in the sea, captured a little sunshine and carbon, then died and sank to the bottom. There still are trillions of them floating about in the oceans today, sucking up carbon dioxide, pocketing the carbon. Over the millennia, so many have died and plopped on top of each other, the weight of them and the water above has pressed them into a white blanket of rock, entirely composed of teeny skeletons. ... The White Cliffs of Dover are all chalk, piled hundreds of feet high."

Nature never ceases to astound.