you know, with humans it is hard but not too hard  
“once a month you lose a drop of blood, that is your body cleaning your self, nothing wrong there”  
but with a pony?  
“ok so your body feels hot, you are aroused all the time and your genitals feel on fire, for the whole time it happens you want to fuck anything that moves…”  
ech.

Rarity: “It’s time for the talk. When two ponies love each other very much…”
 
Meanwhile at Sweet Apple Acres…
 
Applejack: “You know that time of year when everypony gets really horny?”  
Apple Bloom: “Yeah?”  
Applejack: “That’s estrus.”  
Apple Bloom: “But why?”  
Applejack: “Ya see, when us mares are ready to make foals, our bodies start producing chemicals that induces an intense state of arousal and sexual receptiveness. Our bodies also produce a pheromone that gets the stallions all excited. And when you mix a mare in heat with a horny stallion, hoooooowee~!” <3  
Apple Bloom: “Ah get it now. I can’t wait to tell the others!”

@HeavyMetalMare  
Badass SN :D

Rarity gives her a book “The Loveliness of Mare” : nothing but pictures of flowers

Believe me Sweetie, as soon as you hit it you’ll know exactly what it is.

You’ll know when you’re older, Sweetie Belle. It is inevitable.

@DudeManGuy  
That one sentence made me think of an album of songs celebrating “that wonderous time”.

The most wonderful time of the year.