Simon Cowell Simon Cowell

For once, it was Simon Cowell squirming on-stage.

The acerbic American Idol judge blushed and struggled to string together words on Monday's Tonight Show as Jay Leno grilled him on the large diamond ring his girlfriend, Idol make-up artist Mezhgan Hussainy, is wearing.

"Are rumors you're engaged true?" Leno asked.

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"Are they true?" Cowell said after a pause. "Well, I do have somebody in my life now, Jay, yes. And I kind of made a decision this year to make somebody happy. ... It's called giving back."Leno called out Cowell for blushing, to which the Brit said he found it "quite uncomfortable talking about my personal life like that."Undeterred, Leno asked, "Where did you, or may have, gotten engaged?""I was asleep, I think," Cowell said. "No, look, I bought her the ring in London."Pushing for wedding details, Leno angled for a spot in Cowell's wedding party, asking if he could be the best man. Cowell, who said he wants a wedding "within the next 10 years," told Leno he "can do the flowers."

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Hussainy, 36, then briefly joined duo on stage and was greeted by a kiss from the 50-year-old Cowell, who remained equally coy when talk turned to having children."I'm kind of torn because I'm a bit too old to have kids, but then again I think it would be important to have a lot of me's around in the future," he said.