This is a test of commonsense. Complete the next sentence:

How to sharpen lawn mower blades
Disconnect the spark plug and power source.
A lawnmower can cause serious injury if it starts while you are working on it. Always disconnect the spark plug and the power source (outlet or battery) before disassembling your mower.
The spark plug can usually be disconnected by removing a prominent wire on the side or front of the lawnmower engine from its metal mounting. Once disconnected, the engine should not be able to start.