trial and error

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A method of reaching a correct solution or satisfactory result by trying out various means or theories until error is sufficiently reduced or eliminated.

from the GNU version of the Collaborative International Dictionary of English.

• The method of discovering something desirable by trying many different things until one works.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The process of finding a solution to a problem by trying many possible solutions and learning from mistakes until a way is found.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun experimenting until a solution is found