Check The Cranks On A Bike
Modern bikes usually have cranks made of aluminium, they are mounted on an axle made of harder material, usually steel. If there's any play between the cranks and the crank axle the soft aluminium will soon get worn to the point where it's impossible to lock the cranks. If the cranks are loose don't ride the bike.
Riding a bike – even a short distance – with a loose crank may do damage that can only be repaired by replacing expensive parts.