the "minhag" of adding a bit of water to the cup before putting in the oil for your chanukah menorah

I don't buy those pre-filled cups with oil for my menorah.  I do things the old fashioned way and pour in the oil myself.  Unlike the pre-filled cups, which to the best of my recollection, are completely filled with oil, I always add a little water to the bottom of the cup.  

I never thought of this as a minhag; I thought it's just common sense because I don't want the fire to burn down and char and blacken the cup.  Lo and behold, take a look at this Baal haTurim and the notes in the Oz v'Hadar Mikraos Gedolos:

Baal haTurim writes that the three wells which Yitzchak dug correspond to 1) the Babylonian exile; 2) the complaints of Haman against the Jews who were trying to rebuild the Mikdash; 3) the Greeks, who tried to prevent us from fulfilling the mitzvah of p'ru u'revu by banning the use of mikveh.  

The Greeks failed, says Baal haTurim, because a miracle happened and a spring emerged in every person's home so that each person had a personal mikveh to use.  This is why, says the Imrei Emes, the text of the bracha is "she'asah nisim," in the plural, not nes, in the singular.  The bracha commemorates not only the nes of the oil on Chanukah, but also the nes of the mikveh (but we say the bracha on Purim as well?  You got me.)  

And this is why there is a minhag to add a little water to the bottom of the cup before you fill it with oil.  It's not just to prevent the cup from being blackened, but ir's an allusion to this miracle of the mikveh.