A six-year-old boy is seriously ill in hospital after being involved in a quad-bike crash at Watnall in Nottinghamshire.
He was with other children riding the machines on waste ground at a brick-yard on Long Lane.
The boy was taken to the Queens Medical Centre with serious head injuries.
He is now "very poorly but stable" after surgery.
Inspector Paul Selby said: "Two of them were in collision with each other.
"One bike was ridden by the six-year-old, one ridden by an adult.
"It would appear the injury was caused by something going through the front of the helmet."