Gears - wherever you find them - is a way of trading strength for speed, or the other way around. On a bike they let you keep pushing the pedals at a comfortable pace and effort uphill as well on the flat.
Changing Gears Flagstaff and the Mountain Bike - 2011 was released on: USA: September 2011
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?
No one gear is all you need, go singlespeed :)
The idea behind multiple gears on a bicycle -- whether it's an older "10-speed" bike or a modern mountain bike with 24 gears -- is to let you change the distance that the bike moves forward with each pedal stroke.
Well, there's one bike called the autoshift, which looks a bit MTB-like and is supposed to do that, but a regular bike shouldn't
It depends on the bike if the engine of bike is heavy it has more gears. Usually 5 gears are max gears.
2 gears
The essential features of a hybrid bike are a combination of features seen in regular bikes as well as mountain bikes. You have changing gears as well as easy to adjust chains.
There are MTBs with 20" and 24" wheels for smaller riders, which probably is the closest thing to what you're describing. These wouldn't be BMX bicycles though, since BMX bikes don't have gears and have a taller handlebar than a mountain bike.
depends on the model. from 2-7 gears
MotoGP bikes have 6 gears.
Tullio Campagnolo made the first derailleur, but no one really invented MTB gears. Gears were already available for road bikes, so people started using those and then gradually changed them to work better for MTB-type riding.