"The last time we were here, we played Burning Questions, and I'm not sure I believed a lot of your answers," Ellen said to Simon, before introducing a new game involving a lie-detector test, which aired on Tuesday's show.
Sitting opposite each other with lie-detecting microphones close to their faces, DeGeneres asked Cowell a series of questions related to both his personal and professional lives. Before the grilling even began, Cowell said he had "road-tested" the game with his 5-year-old son, Eric, adding, "It does work." He was unknowingly setting himself up for failure.
"Simon, do you think looks are more important than personality?" DeGeneres began. Cowell paused, let out an "ugh, God," then asked, "In work or personal life?"
"Just a question," Ellen replied with a smirk.
Laughing, Simon managed to let out a no. As he probably expected, the game detected he was lying. "This is brilliant," he said with a chuckle.
"Would you ever blame an employee for a mistake you made?" Ellen then asked.
"Yes," her guest replied with no hesitation. The detector showed no lie.
"Do you think anyone in this room has a crush on you?" Ellen asked.
"Yes!" Simon said with no hesitation, this time adding a dramatic head nod. Once again, the man of the hour was telling the truth.
Giggling to herself and shuffling through cards, Ellen said to herself, "These are all yeses." She looked at Simon and asked, "Do you ever pretend to be sick to get out of any plans?"
Unapologetically, he said he did, and the detector showed no lie.
Question No. 5: "Do you think that you're the smartest person in this room?"
Cowell was almost shocked himself when he let out a no, prompting DeGeneres to bust out laughing. "That's a lie," she told her guest, before the detector corroborated her theory.
The audience was in stitches, and Simon looked genuinely surprised he was caught lying. Or perhaps he was surprised because he wasn't lying? We can't be sure.
"Are you a good kisser?" DeGeneres asked Cowell, who barely let her finish the question. "Yes," he said with an obnoxious smile, which the former "American Idol" judge has managed to perfect.
"Come on," he begged the machine, before it revealed he was, indeed, a good kisser (at least according to his own belief). Unsurprisingly, he applauded himself.
"Do you ever lie about your age?" DeGeneres asked.
"Not anymore!" Cowell replied candidly. So Ellen clarified, "In the past, have you lied about your age?" Again, Simon's answer was no, but the detector picked up a fib.
"Are you always an honest person?" Ellen asked. Breaking his look of genuine offense and confusion at the machine's previous revelation, Cowell said no and let out a hearty laugh. He was telling the truth.
Later, the dynamic twosome sat down for a chat. Cowell explained he has a producer who has a way of telling him he looks terrible without using so many words. (That's more than Simon can say!)
"So I went to see this doctor in London," he explained, "and we did some tests, and then a month later, he said, 'You have the worst diet I've ever met from any patient.'" Simon asked his doctor which of his three vices -- "the smoking, the drinking, the diet" -- he had to "drop."
"He said, 'The diet.' So I said, 'Great, I'm in!' The following day," Cowell said. When DeGeneres asked what his new diet consisted of, he replied, "No dairy, no sugar, no bread, no gluten, no red meat." Rough. He explained his wildly energetic son motivated him to get healthier, though he's grateful he didn't have to give up everything he loves.
Simon turns 60 on Oct. 7, and even though he's feeling "great," the television mogul says he's "never actually liked celebrating my birthday." So he asked his partner, Lauren, to keep things casual and quiet -- "no birthday cake."