"Yeah! He stores electricity in his cheeks and has to let out out sometimes." Ash cups his hand over Pikachu's cheek and gives it a nice solid rub. The effect is immediate-

"Pika-chuu!" Pikachu lets loose a jolt of electricity- though it's not a very strong one it's strong enough to leave Ash momentarily stunned, hair standing on end and fingers twitching slightly.

"Sorry buddy, I forgot you don't like that."

"Pikapi." Pikachu says, sounding rather annoyed.