The Grim Adventures of Billy and Mandy S04 E18 Puddle Jumping

"It's Good Grim Fun!"

When Billy jumps into a supernatural puddle (a Cosmic Sinkhole, according to Grim), it takes him to the home of Granny Applecheeks, who wants to bake him into a pie. It is up to Mandy and Grim to save him if the puddle can take them to the right place.