bathroom aerator doesn't want to screw tightly into place
Take a rag soaked in lemon juice, vinegar or CLR and shove it up into the faucet against the internal threads for a few hours. Keep re-soaking the rag. Then brush the threads with a toothbrush and reinstall the aerator. Remember to use the washer that came with the aerator. Oh, don't use the toothbrush on your teeth...