Mud, glorious MUD!

His sister wasn't interested in the mud.

 This did not stop him from enjoying himself solo

 What I found most entertaining was how he kept saying "Yuck, wet, ucky!"  Then playing some more.

 She did eventually venture over

 They decided that mud is just more fun with two than one

 This cold squishy experience must be shared in order to appreciate it fully.

 Yeah, I'm at a loss for why children think this is so great.

 I certainly could have done without this mess.