Inspect the tire: It's important to find whatever caused the flat and remove it. If you don't, the sharp item might still be in the tire where it'll just pop your new tube. To find it, remove your glove (or use a rag), and run it around inside the tire in both directions (photo). If something sharp is still stuck in your tire, it'll snag the glove. Remove the nasty. If you can't find anything, it's likely it got knocked out during the disassembly procedure.