How does a drinking straw balance work?
  We examine working mechanism of a drinking straw balance. Unlike the usual horizontal level balances which rely on the pivotal rotation, a drinking straw balance shows rolling motion. Because of this rolling and relative position between the pin and the center line of a straw, a new equilibrium state is possible after placement of a test mass. Relative errors between the measured values by a drinking straw balance and an electronic scale are order of 22%. Minimum value of a measurable mass by a straw balance is order of 10 mg. 
Hyunbyuk Kim, Jungwoo Park
physics.pop-ph