It’s through tickling

Mary: I get to tickle you while I do it!

Marylin: W-What!?

Mary: you herd me~

Marylin:… f-fine!

One drive later—

The two were home. Mary pushes Marylin to her room and pulls a box from under her bed. She pulls out some handcuffs.

Marylin: M-mom…?

Mary: get on the bed sweetie ~

Marylin complies and gets tied in an X shape.