Just to add an addendum to Capt Wal's post. When doing your preflight checks, make sure the manual u/c extension handle is stowed in it's clips. If it isn't fully stowed and you select gear up you can do extensive and very expensive damage to the gears in the u/c gearbox.
Essentially there is a large centre gear with two smaller gears, one driven by an electric motor attached to the u/c selector switch and the other is driven by the manual extension hand crank. When you unlatch the manual extension handle it moves the electrically driven gear away from the centre gear and connects the hand driven gear.
When the manual extension handle is not fully extended you can get a situation where both small gears are still partially connected to the centre gear, usually by 1 - 3mm and the teeth on all of the gears can be stripped. The gears are stainless steel and very expensive.