Remember when, during his tumultuous trip to London, Justin Bieber was spotted out and about wearing a gas mask -- and no one knew why? (If you'll recall, this was way before his hospitalization for breathing issues, so that wasn't the reason -- though it was one heck of a coincidence.)

Now we do!

A source told Us Weekly, "The gas mask Justin has been wearing around London is a joke between his friends, but people are starting to take it seriously. Justin's close friend Julian Keyz also wears it to pretend he's Justin and throw people off as they're the same height."

Keyz, a young songwriter/producer also known as Julkeys, joined Biebs across the pond for his trip. Without the mask, they really do look nothing alike!

"All his friends take turns wearing it as a joke to throw off the paparazzi," the source added. "It's not a serious thing, they're just having fun around London with it."

Considering Biebs was still a little short of breath at what turned out to be his comeback show at the O2 Arena on March 8, he may want to consider wearing one when he performs!

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