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Inside many of your cells are tiny structures that carry their own DNA, separate from the DNA in the cell's nucleus. They are the descendants of bacteria that were swallowed more than a billion years ago, never got digested, and never left.

Take sea otters away from a stretch of coast and the kelp forest can vanish, leaving bare rock carpeted in spiny urchins. The odd thing is that the animal holding the forest up never eats the kelp at all.

Go over something three times in one evening and it feels nailed down. Spread the same three sessions across a week and it feels shakier. Yet the spread-out version tends to win when it counts. The secret ingredient is a little bit of forgetting.

Dub the sound of one syllable onto a film of a mouth shaping another, and people report hearing a syllable that's on neither track. The trick is old, simple and stubborn, and it shows sight reaching into hearing.

Run your own fingers along your ribs and almost nothing happens. Let someone else do exactly the same thing and you squirm. The difference isn't in your skin. It's in a quiet forecast your brain makes every time you move.

Some chains of volcanic islands get steadily older as you travel along them, like a timeline laid across the ocean. The reason is that the source of the lava stays roughly where it is, while the plate above it creeps past, punching out new volcanoes as it goes.