Erdös and Problems
“Problems have always been an essential part of my mathematical life. A well chosen problem can isolate an essential difficulty in a particular area, serving as a benchmark against which progress in this area can be measured. An innocent looking problem often gives no hint as to its true nature. It might be like a ‘marshmallow’, serving as a tasty tidbit supplying a few moments of fleeting enjoyment. Or it might be like an ‘acorn’, requiring deep and subtle new insights from which a mighty oak can develop.”
Quoted from ‘Some of my Favorite Problems and Results’, in The Mathematics of Paul Erdos I, R L Graham and J Nesetril (eds), Springer-Verlag 1991.