So I got myself a $20 thrift store mountain bike for my "learner" build. I managed to get a new tire and tube mounted on the front without problem but now I am working on the rear wheel. I removed the axle to grease the bearings and now I realize that I need to remove the sprocket so I can reinstall the axle. It looks like it takes some sort of spanner tool. I'm sure most of you have had some experience with this sort of thing. What is the tool called and how much can I expect to pay to get one? I guess I'm learning now.