_Welcome_To_My_Hell_ -
In the summer the Dreemurr siblings play with water guns. Once Frisk and Asriel made the mistake of ganging up on Chara and drenching her. So she forces them to stay underwater in the pool Until they almost pass out for revenge

Frisk: Thank you so much Sans for taking us to the lake for a picnic!

Asriel: Yeah, Thanks Sans!

Sans: No prob kiddo's... Hay you know, your sisters comin' back with the picnic basket any second now. I got a few water guns in my backpack over there. Why not give her a surprise when she comes back?

*Frisk and Asirel run off with the fully loaded water guns down the lake to wait for Chara*

Chara: *calls* Hay guys, Im back! *sees no one near the picnic blanket ahead* ... guys?

Frisk and Asirel: *jump out from the trees* SURPRISE ATTACK

*moments later*

(oh boy guess what, a bonus sketch I make of after the events :D)