For that helmet to protect you, it needs to be worn properly.
"A helmet needs to be both comfortable and snug," says Patty Davis of the U.S. Consumer Product Safety Commission.
"It needs to be level on the head, not tilted back or pulled too low over the forehead. It shouldn't move in any direction - front-to-back or side-to-side. And make sure that chin strap is securely buckled, so it doesn't fall off if you have an accident."
Always wear a helmet, even if you're going a short distance, even if you're not going that fast.
"You can have a substantial injury to your head even when you're riding slowly," says Davis.
When you buy a helmet, make sure it's meets the CPSC helmet standard. New helmets with the "ASTM," "ANSI" and "Snell" labels do meet that government standard.
By the way, bike helmets can also be worn when in-line or roller skating.
CPSC: Which Helmet for Which Activity?
Seattle Children’s: Bike Helmet Safety