Spark Plug Change - Rapped Knuckles

New spark plugs for the car today as there has been lots of troble starting it up recently. Spark plugs are easy enough to change and before I even started I knew there was going to be some pain involved and I was right. The twist of the wrench took my knuckles to hit the carburettor and remove a layer of skin! I just don't know why this always happen when changing spark plug.