24. Rear Wheel
How To: To reinstall a back wheel with no gears, put the chain onto the rear wheel's freewheel or cog. It's easiest to put the chain first onto the gear on the rear wheel, then route the chain on to the sprocket at the cranks. Keep the wheel centered in between the frame. Additionally you'll also need to keep the chain tensed as you tighten the wheel. Your chain should have 1/2-inch to 3/4-inch of vertical slack once your wheel is tightened into place. This can be hard to measure. Basically, you want the chain to have enough slack in it so that it runs smoothy over the sprockets without binding, yet won't fall off and derail.
To reinstall a back wheel with gears, push down on the bottom guide pulley of your rear derailleur. This will open a gap in the chain loop where you want to install the wheel. Slip the rear wheel into the opening in the chain loop you have created. Guide the chain onto the smallest cog (which is where you shifted the chain to earlier before you removed the rear wheel). Place the wheel into the dropouts of the frame.