Foucault pendulum at the Panthéon in Paris, France (© Adolf/Adobe Stock)

In 1851, when French physicist Léon Foucault performed his pendulum experiment in Paris, he became the first to prove what many scientists of his day already suspected—that the Earth spins on an axis. He conducted his first experiments 170 years ago, in early January 1851, with a relatively small prototype in the cellar of his home. Just a month later, Foucault performed his most famous pendulum demonstration, using a 62-pound spherical weight attached to a 220-foot wire, which was hung from the dome of the Panthéon, a Parisian monument. As the pendulum swung back and forth in a fixed plane, the pointed end of the weight traced lines in a compass-like circle of sand below it. As time passed, the angle of these lines began to change, demonstrating to onlookers that the Earth itself was rotating underneath the pendulum, and by extension, everyone watching was rotating as well, spinning on the surface of the great blue marble around its axis.

Spanish shawl nudibranch on kelp off Santa Catalina Island, Channel Islands, California (© WaterFrame/Alamy)

Today we're meeting one of a motley group of sea slugs called the nudibranchs (rhymes with 'thanks'), known for their unique, often complex shapes and neon-bright colors that help discourage predators. The Spanish shawl's fire-orange mane is made up of tendrils called cerata that mainly act as gills. But that mane also retains venom from the slug's prey—sea anemones—treating any would-be devourers to a painful sting. Should a ravenous sea star disregard these defenses and get too close for comfort, the Spanish shawl has a Plan B: By flapping its whole 2- to 3-inch body like a gelatinous wing, the nudibranch can flutter into open water for a quick escape.