With a child sitting on the floor or chair, I pick up one foot and hold by the ankle in my hand.  I take one finger, run it down the center of the bottom of the foot (from toe to heel) and say “Peanut Butter”.
Then I repeat on the other foot, saying “Jelly”.
Then I say “Sandwich”, and press the two foot bottoms together, holding them in my hand.  I pretend to eat the feet with my mouth while squeezing with my hands.