Method 2:
For marked thermometers write number of parts with mercury.

This looks interesting.

Edited by rakesh_rai 2011-07-07 4:40 PM

Also an option, number of empty thermometers or completely filled ones

Edited by utkaarsh 2011-07-07 7:06 PM

That doesn't work in general because in most cases you don't need to solve the puzzle completely to know #of empty/full thermos.
Secondly, global counting is something error-prone (e.g. #of horizontal bridges in Hashi). I think we should avoid answer keys like this.