>>>URBAN LEGEND ALERT!!!!<<<
I seem to remember that when the "athletic" shoes with the lights built into the heel first came out, there was concern because one of the chemicals that made the lights work was Mercury. There were terrible visions of one of those light pockets breaking open and Mercury being spilled everywhere.....
Of course, there was never any documented proof...just the "my brother's best friend's cousin that is married to the guy who delivers boxes that the shoe company uses to put the shoes in" told me so.
