Kate Middleton also revealed her favorite emoji.
Prince William's favorite emoji may not be what you'd expect. Earlier this week, the Prince of Wales and his wife, Kate Middleton, appeared on BBC Radio One's Going Home, to promote World Mental Health Day.
Amid their discussion, hosts Jordan North and Vick Hope asked which emojis the couple used the most.
"Is this a clean thing or is this a family one?" William, 41, questioned, before delivering his surprising response.
"I've been told not to say the aubergine [eggplant] so I've got to pick something else," he said. "It would have been the aubergine but I'm saying now -- because I've got to be all grown up -- it's the one where the eyes go up and down and the mouth's out. What's that one? The slightly crazy one."
For her part, the 41-year-old Princess of Wales said, "Mine's probably got to be the heart with then the crying emoji, the sort of hysterical laughing [one] when things have gone wrong."
During their chat, the royal couple also discussed their dinner plans, revealing they'd likely be dining on curry that evening.
"I can't do too much spice, I start sweating. It's not attractive," William admitted, before his wife noted, "Whereas I like the spice, so I have to sort of cook the curry and then add the spice, extra spice, at the end."
"She has to bring it in gently because otherwise I get too sweaty," William quipped. "It's not a nice sight!"
"This is a fleeting visit, and the couple tend not to travel abroad together unless they have to, because, of course, they're juggling childcare and schooling for three children," royal expert Katie Nicholl told ET of William's U.S. visit for The Earthshot Prize. "This is William's project... This summit was something that William planned to always do on his own."