This is an easy one: we all blink.
If you are a hamster or a tortoise (hi there, guys!) you'll only blink one eye at a time. If you're a bird, a reptile or a shark (does anyone make an underwater computer?) then you'll blink with your third eyelid, which comes across vertically and looks really quite spooky.
We blink for several reasons. We blink when something comes close to our eyes; we blink to keep our eyes moist (not you, sharks); oddly, we also blink when we hear a loud noise.
We blink less often when we're very young, and when we focus on one thing for a long time. This is why reading can make the eyes feel tired. Perhaps we should have BLINK! signs in writing as reminders: or perhaps a full stop (period) could take on this extra function.
Well, it's a thought, anyway.
Thing To Do Today: blink. This word is related to the Middle Dutch word blinken, to glitter, and to the Danish word blinke, to wink.