According to the recently discovered 'Law of Urination', mammals, ranging in
size from mice to elephants, take, on the average, 21s to urinate. We attempt
to gain insights into the physical processes responsible for this uniformity
using simple dimensional analysis. We assume that the biological apparatus for
urination in mammals simply scales with linear size, and consider the scenarios
where the driving force is gravity or elasticity, and where the response is
dominated by inertia or viscosity. We ask how the time required for urination
depends on the length scale, and find that for the time to be independent of
body size, the dominant driving force must be elasticity, and the dominant
response viscosity. Our note demonstrates that dimensional analysis can indeed
readily give insights into complex physical and biological processes