Sugar cube is filled with liquid (coffee), only if corner is sinked for few millimeters

Sorry for quite stupid question, but I could not find answer for it (perhaps my question is wrong).


I noticed one thing about sugar cubes. When I sink one corner of cube in coffee, just for few millimeters, after a few seconds it's already turning brown (filled with coffee, so to say). Another thing, if I sink it for a short period of time and take it out, coffee liquid stops spreading, if I sink it again it continues.


Why this is happening?


