Octopuses have three hearts, and two of them stop beating when they swim.
When an octopus swims, the two hearts that pump blood to the gills stop working, which is why they prefer crawling. This is also why they get tired very quickly while swimming — it literally starves their body of oxygen.
Bonus fact almost nobody knows: their blood is blue because they use copper (hemocyanin) instead of iron (hemoglobin) to carry oxygen.
Did you know that a single bolt of lightning contains enough energy to toast 100,000 slices of bread?
(And yes, scientists have actually calculated that—about 1 billion joules per typical bolt. Now go forth and casually drop that at your next dinner party.)
Did you know that a single teaspoon of neutron star material would weigh about 6 billion tons on Earth?
That's because neutron stars are the collapsed cores of massive stars after a supernova. Their gravity is so intense that electrons and protons are crushed together into neutrons, packing the mass of the Sun into a sphere just 10–20 km across. The density is so extreme that a sugar-cube-sized chunk would outweigh Mount Everest.
Bonus weird fact: they can also spin up to 700 times per second, and some of them act like cosmic lighthouses, blasting out beams of radiation that we detect as pulsars.