For a couple of months up until last week, my piano's "F" key (the one in the octave beneath middle "C") often stuck, and sometimes even the key or keys adjacent to it would stick also.
Someone told me that their piano had had keys that stuck, but the person working on it simply cleaned off the keys and it worked fine after that. Well, my mom asked me to go try to do something to fix it around last week, so I removed all the books and other things from the top of the piano and opened the top. I looked inside, found the stuck "F" key, and I found--
a pencil, stuck in the piano. Pretty sad to think that all that time, the stuck key was caused by a pencil. So, always remember to check things yourself before calling a professional to repair a problem.