Related : Kim Kardashian & Kylie Tell Kris Jenner Who's Poisoning Her

The Kar-Jenners are up to their old tricks again!

In this clip from Sunday's all-new Keeping Up With the Kardashians, Kim Kardashian and Kylie Jenner reveal who's been poisoning their mom Kris Jenner, but they're not doing it because they're concerned for her health.

"Oh my God, we have to f--k with mom," Kim tells Kylie. So, the sneaky sisters call their mom to break the big news.

"Hi mom," Kim says. "It's me and Kylie. We figured the whole thing out." "What'd you figure out?" a curious Kris asks.

"That Kourtney has been poisoning you this whole time," Kim reveals.

The revelation comes as a bombshell for the momager who had just tested her drinking water for poison with daughter Kourtney Kardashian.

"Stop. No way! I just was with her. Why would she do that?" Kris wonders.