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Worried about those popping and cracking sounds from your knees and elbows?Graham Hughes/The Canadian Press

Worried about those popping and cracking sounds from your knees and elbows? Got an eyelash stuck in your eye? Well, just step into Dr. Wisdom's surgery and all will be well. Now, where's Egor with the leeches?

THE QUESTION: Why do knees and elbow joints "crack" at certain times, such as when we stand up from sitting in a chair? David Kerr of Toronto wants to know.

THE ANSWER: "Freely movable joints are covered by a capsule that contains synovial fluid, which acts as a lubricant and also contains nutrients for the adjacent bone surfaces," writes Carla Hagstrom of the Gerstein Science Information Centre at the University of Toronto.

"A variety of gases are continuously dissolved in this fluid," she says. When the capsule is stretched, for example by "cracking" a knuckle, the pressure inside the joint is lowered and a vacuum is created that is filled by the gas previously dissolved in the synovial fluid. This creates a "bubble" that then collapses, producing the characteristic "popping" or "cracking" sound.

She adds that it takes a while until these gases are redissolved in the synovial fluid, which explains why knuckles cannot be recracked immediately.

THE QUESTION: Staying with matters of the human body, Joan Phillips of Toronto asks, "When you get an eyelash in your eye and you can't get it out, what happens to it?

THE ANSWER: "Eyelashes routinely get loose and lie on the surface of the eye, attracted there by the tear film and held for variable periods of time by the natural, oily tear-film materials," writes Rod Morgan, an Edmonton ophthalmologist.

He says it's easy to experience mild irritation from lashes that get caught in the skin folds on the surface of the eye and the lid edges, but unless the lash penetrates through the skin on the surface of the eye, the brief irritation disappears as the tear film, blinking lids and eye movement flush the lash out onto the lid surface, where it falls away from the skin after the lash dries.


Last week, we mistakenly attributed a question to Roy Overton of Sarnia, Ont., that was actually sent in by Wes Libbey of Cornwall, Ont. Mr. Libby's question was: Why do public washrooms have low urinals for little boys but no low washbasins? We're still open for answers on that one, by the way.


  • Okay, this question really is from Roy Overton of Sarnia. He says he has noticed that ATM machines dispense new, or almost new, $20 bills and wonders how many of these new bills are issued weekly, and are the same number of old bills taken out of circulation?
  • Norman Holler of Whitehorse, who says he has been "plunking away at a muffin-crumb-impregnated keyboard," wonders why regular computers can't have flat-surface, touch-sensitive keyboards like the ones in iPhones and iPads.
  • Jonathan Haldane of Mississauga says his five-year-old son, Aidan, stumped him with this question: If fire needs oxygen to burn, and there is no oxygen in space, how is the sun able to burn?

Send answers and questions to Please include your name, location and a daytime phone number.