How do I reduce slack in the chain of a single-speed bicycle? I have a single speed bicycle that has some slack in the chain and threatens to come off.  How can I reduce this slack?
If you have horizontal dropouts (the wheel axle slides into the frame from the back of the bike) the dropouts have long slots which will allow you to pull the rear wheel back and take slack from the chain.  Be careful to ensure when you torque the wheel nuts down the wheel doesn't slip from being aligned straight, or the chain is running at a slight angle.

If the chain is so stretched to the point where it's falling off, you really should consider replacing the chain.  You can get away with removing a link to remove the slack but a worn chain will be prone to sticky links and popping off unannounced.