Pinkie Pie swear/promise used in the episode "Green Isn't Your Color" of MLP:FiM. It goes like this: Cross my heart (cross your heart) and hope to fly (mimic the act of flapping wings by moving wrists) stick a cupcake in my eye (pretend to smash said pastry into your eyeball). Said by both Pinkie Pie and Twilight Sparkle.
Pinkie Pie: Cross my heart and hope to fly, stick a cupcake in my eye! (Pinkie Pie Swear)
Twilight Sparkle: Cross my heart and hope to fly, stick a cupcake in my--AAH! (Pinkie Pie Swear fail)