Though the thought of that shop and it's exotic wares is enticing, you can't get the image of Derpy's bouncing rump out of your head. You certainly wouldn't mind a closer look. She isn't out of sight just yet, thankfully, and neither is that voluptuous flank. You keep your distance, not wanting to tip her off, even if she is a little inattentive. You're not so far, though, that you can't hear her singing,
"They can feel it all over, they can feel it all over ponies go, doo doot doot doodle doo da da cock a doodle..."
She's terribly off-key, but it's almost cute. Her mailbag is hanging limp as she starts heading home, with you following incognito. She stops at one of the doors, and retrieves a small key ring. Derpy tries her first key, but fails, and has to try the next. And the next. She's there for quite a bit, before she finally calls out,
"Muffin! Mommy's here, and she kinda forgot which key is the one that opens her, um, door."
A unicorn filly pokes her head out of one of the upper-story windows, realizes it's her mother there once again, and pulls her head back inside. You her some quick clopping, and the unicorn opens up the door for her.
"Thanks Dinky." Derpy says, abashed, "Sorry I was late, Muffin, did you see the meal I left out?"
"Yeah mom, no big deal." Dinky says, leading her mother inside. You take this as your opportunity to slip into a good viewing location. Around the side of the house there's a neat little hedge with a view inside, to what you assume is the living room. Dinky and Ditzy enter, and you press an ear up against the window to hear their conversation.
"...just doing my spelling homework, Miss Cheerilee gave us a big packet today. Not too tough though."
"Aw, that's my smart little muffin!" Derpy says, climbing up onto the sofa, "But before you go, do you think you could lend me a hoof?"
"Sure mom, what do you need?" Dinky turns around and finds her mother's backhooves in her face.
"Give your poor mom a hoof rub?"
Dinky leaps back, pinching her nose, "Ew, mom! Your hooves smell awful, were you walking around all day?"
Derpy blushes, "Yeah, sorry... They're not letting me fly, not since the piano incident."
"Mom, the smell is literally making me sick, I.. Ugh.." Dinky looks from the rancid hooves, to her mother's pleading eyes, and gives in, "Fine. I'll rub 'em. But only a little."
Dinky reaches her hooves around her mother's sole and starts kneading it. Unfortunately, with both her hooves occupied she can't block out the smell. You're watching, quite happily, from the window. You're not sure if it's her daughter's discomfort or Derpy's pretty little body, but it's got you hot and bothered. Dinky moves onto the next hoof, her nose becoming blissfully numb to the smell. Derpy is definitely enjoying herself, laying back and occasionally moaning. You think she's enjoying herself a little too much, in fact.
"Oh my Celestia!" Dinky shouts, "Mom, did you pee?"
Derpy looks down to her crotch and blushes. She grabs a pillow and puts it over her soaking labia, pushing her daughter away. Dinky sat on the floor for a bit, shocked with her mother's strange behavior.
"Go, uh, go do you homework!" she shouts, and Dinky panics, dashing upstairs before something else happens.
Derpy pants a little bit, terrified that she was almost caught by her daughter. She listens for a bit, and hears a door slam upstairs, then sighs. Coast is clear. She moves the pillow away and you can clearly see her pussy, glistening with her feminine juices. She raises a hoof and...