If there’s one thing we You Docs would swear on our diplomas — no research needed — it’s that you, us and everyone from China to Chicago has walked into a room and had no idea why you’re there. You curse having the memory of a mosquito (or just curse).

Well, get this. Two memory researchers have determined why it happens. It’s called (try to remember this) the doorway effect.

Example: You walk across the room to get this newspaper. No problem.

But if you walk through a doorway to another room to get it, zap, your memory is Windexed. You arrive clueless.

Why? When you cross the room, your surroundings don’t change.

When you go through a doorway, you enter a new space. Your brain auto-purges info you needed in the old room (go get the newspaper) so it can cope with the new room’s demands (watch out for the cat!).

Way back, this was handy if you were leaving the safe cave for T. rex territory. It still is, if you’re leaving your desk for a meeting, or your cozy bedroom for a slippery bathroom to stop the twins’ water war. But going to get the paper? Not so much.

“So,” you ask, “what’s the fix, You Docs?” We didn’t forget. There just isn’t one, short of never exiting a room without writing down why. Yeah, we’re not doing that either.

Here’s the upside: Every second trip gets you (us, too) closer to your 10,000-steps-a-day goal.

Go get your pedometer. Maybe write it down first.

The You Docs — Mehmet Oz, host of “The Dr. Oz Show” and Mike Roizen of Cleveland Clinic — are authors of “YOU: Losing Weight.” To submit questions, go to www.RealAge.com. A King Features syndicate.