You are limited only by your thoughts.
Here's a story about George Dantzig - the famed mathematician who's contributions to Operations Research and systems engineering have made him immortal. As a college student, George studied very hard and often late into the night. So late, that he overslept one morning, arriving 20 minutes late for Prof. Neyman's class. He quickly copied the two maths problems on the board, assuming they were the homework assignment. It took him several days to work through the two problems, but finally he had a breakthrough and dropped the homework on Neyman's desk the next day. Six weeks later, on a Sunday morning, George was awakened at 6 a.m. by his excited professor. Since George was late for class, he hadn't heard the professor announce that the two unsolvable equations on the board were mathematical mind-teasers that even Einstein hadn't been able to answer. But George Dantzig, working without any thoughts of limitation, had solved not one, but two problems that had s...