Twelve vehicles with sounds heard on the street every day: the ambulance siren, the train whistle, the bicycle bell. It is many children's favourite category, and the one they connect fastest with what they hear when they go out.
Car, motorcycle, train, airplane, helicopter, boat, ambulance, fire truck, police car, tractor, bicycle and truck.
That is exactly the part that trains the ear. Ambulance, fire truck and police sound alike on purpose: you have to listen closely to tell them apart, and if the child gets it wrong they can listen again and keep trying in the same round.