You can get a really finely twisted piece of paper towel or tissue and kinda rub around. Maybe have a paperclip inside the paper towel, y'know, so you have a bit of support. Of course, if it is liquid, it'll dry out over time - I had this happen recently, so I just let it sit for a while and it worked again after a while. Note that my friend did the paper towel trick to the same result, so it's up to you which is better.
