A man in his 60s is in hospital after being hit by a brick hurled through a bus window.
The passenger suffered a serious head injury and is now stable in hospital, Merseyside Police said.
They are appealing for information on the incident at around 11.05pm on Thursday on West Derby Road, Liverpool.
Detective Inspector Alan Martin, of Merseyside Police, said: "The sheer recklessness and stupidity of throwing items at a bus or any vehicle is clear. The actions of the offender left a man in hospital and very fortunate his injuries were not more serious.
"Throwing a brick at a bus obviously carries great dangers and I would urge anyone in the community who has information to identify the person responsible to do the right thing and come forward with any information. CCTV enquiries are ongoing to identify those responsible.
"We already do lots of work with local schools to highlight the dangers of this behaviour, but I would also encourage parents, guardians, teachers and friends of any young people around the area to help us spread this message."
The suspect is described as white, aged 16-18, 5ft 7in-5ft 9in tall, of slim build, clean shaven, with short blonde hair. He was wearing a black jacket and black tracksuit bottoms.
Members of the community with any further information are encouraged to speak to officers in the area, or contact police on 101. Anyone with information who does not wish to speak with police directly can call Crimestoppers anonymously on 0800 555 111.