You don't have to, but you'll accelerate faster if you do.
yes it can accelerate.
You can put more gears in the bicycle.
It depends on the bike if the engine of bike is heavy it has more gears. Usually 5 gears are max gears.
2 gears
MotoGP bikes have 6 gears.
depends on the model. from 2-7 gears
A brake shows the car/bike down until it comes to a hult/stop. A gear changes the speed the engine works at depending on what type of road you are driving on and the speed limits. If you are asking about a bike then the gear's change the speed at which you pedal at, gear 7 on a bike makes you pedal slower but you have to push harder down on the pedals, however gear 1 on a bike makes you have to pedal faster and more genitally on the pedals, - both gears really work your muscles... so if you have a bike with gears up to 7 stay on 3 or 4! Gear box = what you use to change the gears on a car. Handle bar = what you use to change the gears on a bike.
Depending on the bike yes most adult sized bikes have gears
if you don't keep your chain and gears on your bike greased it will rust and come off.
Depending on the mountain bike that you have will depict the amount of gears. Most bikes for adults typically have about 21 gears. If this does no answer your question then what are levels?
Changing Gears Flagstaff and the Mountain Bike - 2011 was released on: USA: September 2011
To change gears on the bike, all you have to do (while riding it) is press the B button. You don't have to hold it in, either. ~Lucario