It was at the final fight between Inuyasha and Naraku. It was an hard fight, particularly now, when Naraku hat the Magic stone, but Inyasha fights hard and good, and so he managed to hit the hand of Naraku, that was holding the stone.
Naraku was surprised and released it in his confusion. Because the stone got hit, he flew with a great speed at Shippo and the others, who watched the fight.
The stone flew directly at Shippo, who had his mouth open because he was shocked to see this. And the stone flew directly in the open mouth of Shippo, who swallowed it in confusion.
"Nooooo!" Shouted Naraku, and shot at Shippo to get the stone. But it was already too late, and the stone had build up a shield around Shippo and so the shot of Naraku faded away. But now, it was Shippos turn! A strange energy flowed out his body and he slowly started to grow...