Anon, Derpy's Ball and Bad Mathematics.

>You're out for a stroll.
>You just got done helping Applejack put together some furniture. Fingers really help with the fiddly bits like screws and such.
>You pass by everyone's favourite mailmare, Derpy. She's nudging a blue ball around on the ground.
>You wave when she spots you.
>"Heya Anon! Whatcha doin'?" She asks, as you make your way over to her.
>"Oh just out for a walk." You reply with a smile.
>"I'm playing with this ball!" She exclaims, happily. "It reminds me of bubbles!"
>You chuckle a little until you notice her eye start to twitch and her pupils shrink.
>"Derpy, are you..."
>"The circumference of this ball is 25.4 centimetres, c divided by 2 times pi equals a radius of 4.0425355."
>"Derpy?"
>"Volume equals 4 over 3 times pi R cubed, giving a total volume of 207.5445817 centimetres cubed, rounded to 208 centimetres cubed or 0.208 litres. Roughly."
>You stare, mouth agape.
>"Also it's blue."
>Her pupils return to normal suddenly and she beams up at you.
>"Wanna play!?"
>"I... erm... not just now, Derpy, I have to... go..."
>She looks a little sad for a second.
>"Oh, ok Anon."
>Immediately she perks up and begins humming a happy tune and batting the ball around again.
>"Yay, bubble!" You hear as you beat a hasty retreat.
>Oh, Derpy.


(I'm at least 57% sure my maths is wrong there, but whatever.)