Select the left or right scale by clicking L or R at the top of the screen. Deselect by clicking on background. Click a particle to add it to the selected side. Will default to left if none is selected. The middle number will tell you roughly how much more the heavier scale ways than the lighter scale. You can toggle from the typical particle physicist's eV (electron volt) measurement of mass to metric units by pressing the unit button next to the R button at the top.
