[Avodah] Sefer HaChinuch on why 2 weeks Nidah for a girl and only 1

[menucha]

And of course there is the Gemara in Nida 31b,  - zachar shekulam 
smechim bo mitcharetet leshiva, nekevim shehakol atzuvim ba mitcharetet 
learbaa asar-

I always found that the harder explanation to teach...........
menucha