I just did so yesterday. I was discussing with the ophthalmologist how to estimate when a very opaque bottle of eye drops was near-empty. (I find this user-host

I just did so yesterday. I was discussing with the ophthalmologist how to estimate when a very opaque bottle of eye drops was near-empty. (I find this user-hostile) I've been weighing the bottle on a kitchen scale, and the tare weight is 8 grams, (from the previous bottle) while a full bottle is 23 grams, (from the next bottle) agreeing with the 15 gram (milliliter) stated contents. Assume density of 1.0 g/ml. So, the fun part was estimating the droplet volume. From the measured rate of decline (7 grams in 40 days) the drop size was 45 microliters. (4 drops per day) Easy slide rule math, but the important thing is that the slide rule requires you to understand orders of magnitude (we had a whole bunch of lessons on this in high school) and units. The latter two are winners. Internet lookup only to check results, of course.