Tahi lalat

Had this conversation with Alisa on the way sending her to school this morning.

Alisa : Mama, kenapa bila lalat gigit tak ada benda yg hitam keliling?

Me: Lalat dia bukan macam nyamuk, so dia tak gigit pun so tak ada la bengkak.(I thought she meant the bengkak after biten like mosquito)

Alisa : Bukan tu, yg tahi lalat tu...

Me: Ohhhh! tahi lalat tu jadi bukan sebab lalat gigit, dia memang dah ada masa kita lahir.

Alisa: Dah kenapa nama dia tahi lalat?

Me: Hmmm..mama tak tahu nak jawab apa...

Any good answer for the question??:))