I don't own Naruto.
Sasori was wearing regular clothes to blend in while searching for information on the Yonbi jinchuuriki, when a girl about 7 years old ran up to him.
"Ne, ne, you're Akasuna no Sasori right? I have a question for you."
"Kid, if you know who I am, why are you running to me rather than away from me?"
"I told you, I had a question for you."
"What is it? I'm busy."
"If you can make humans into puppets, have you ever made a puppet into a human?"
Sasori hesitated and looked away for a split second before replying, "No. Now go away, I'm busy."
Out of a nearby alleyway a man in his mid twenties, maybe, ran up to them.
"You liar. You did too. You even gave me reproductive capabilities. Of course I never will be able to be a ninja, but I can at least have kids."
"Oh, really now?"
"Yeah, really now. Why else would a seven year old know you? She's my daughter."
"I thought you died. Too bad, Now I'll have to kill you."
And a somewhat epic fight scene took place. Just as Sasori was cornering the puppet-turned-man, coincidentially named Pinocchio, the daughter grew blade-like claws and tore his body into pieces, leaving the heart and brain intact.
"I never said I couldn't have kids with various puppet powers. Idiot."
They then left, and with what power Sasori had left he transferred his heart, brain, chakra and conciousness into a puppet.
And that is how Sasori became a puppet. He also never tried to make a puppet into a human again. Pinocchio's daughter, Kairyuu, eventually started a clan of ninjas with a kekkei genkai of growing lethal weapons and being poisonous at will.