It's been a mystery for two decades in the making, but we finally know how the toilet paper substitute from the future works.

It's been a mystery that's dogged film fans since "Demolition Man" was first released in 1993: How do you use the three seashells? And now, finally, "Gravity" star Sandra Bullock has given us the answer.

First, a bit of background for those of you who haven't seen the classic action flick: After being accused of crimes committed by Simon Phoenix (Wesley Snipes), super-cop John Spartan (Sylvester Stallone) is cryogenically frozen. He's eventually thawed out in 2032 to help take down a returned Snipes. Stallone teams up with Sandra Bullock's clean-cut future cop, and naturally, cultures clash.

One moment that's stuck with fans for years: after exiting a bathroom Stallone asks why there's no toilet paper, and only three seashells on a shelf. "He doesn't know how to use the three seashells!" says fellow cop Rob Schneider. And then they all laugh, and laugh... But we never found out how the three seashells are used. Until now!

On the red carpet for "Gravity," MTV News' Josh Horowitz asked Sandra Bullock to once and for all settle the greatest mystery in movie history.

"Well, think of a bidet, right?" said Bullock. "There's several processes. You have number one, you have number two, and then the cleanup."

But it doesn't stop there. Turns out the seashells are also musical instruments. "You can use them as little maracas as well," continued Bullock, giving an example of the sort of beat you can drop with the future's answer to toilet paper. "See, it's a musical instrument, and it's a hygiene element!"

Surprised that this mystery which has plagued film criticism for two decades was so easily solved, Horowitz got seriously burned by Bullock.

"It was obvious," Bullock joked. "It was obvious to me when I read it. I thought you were an intelligent man, but obviously not."

Bullock's "Gravity" premieres Friday.