Paul Salomon came up with a beautiful class of problems for encouraging qualitative reasoning, and helping develop an intuitive sense of algebraic operations on an equation or inequality. See them at [url]http://bit.ly/PSimb[/url]
Please note that the scales do not tilt proportionately to the weight difference, and that not every problem has a definite solution.
This was fun to make, so let me know if you have suggestions to improve.
