Misunderstanding the equal sign

Interesting blog post at Cocktail Party Physics on student's misconceptions about how the equal sign (=) works.  If you teach math or science, you should go read it - I found it surprising.  Like the author, I've always thought of the equal sign as a see-saw as well - one that's inviolable.   Turns out many kids have a different view, seeing it in a much more limited way (generally as the "enter" key on a calculator).