Charles Joseph Sarangaya, of Block 24, Lot 6, Palmera Spring II subdivision, Camarin, Caloocan City lost his right thumb to the blast, which took place at around 3:15 p.m.
The reports of the explosion sent Northern Police District (NPD) operatives rushing to the Lords Grace Academy compound.
Senior Superintendent Enrique Robles, Camarin police station commander, said his office received information from a certain Carina Galban, teacher of the Lords Grace Academy, about a boy hurt in an explosion.
The victim, Galban said, was brought to Bernardino General Hospital at Zabarte Road in Camarin, Caloocan City.
Robles said the boy was playing with his sister at a vacant lot behind a school building when he picked up a spherical object covered with black plastic.
Jackson Ora, 21, the schools janitor, who happened to be in the area, played a game of catch with the boy, unaware of the danger posed by the "ball" they found.
As the boy caught the ball, it exploded in his hand severing his finger. Pete Laude