Okay, the story goes that the famous Greek scientist and mathematician Archimedes was lowering himself into his bath, thinking everything was all right, when splish splash he had a great idea and shouted "Eureka!".  How was he to know there was a party going on?  Perhaps there are skeptics about this story, but they will please keep their reservations to themselves.

His great insight went like this: as I lower myself into the tub, the water rises.  The amount the water rises isn't proportional to my weight, or to my intelligence, or to my incredible wit.  Instead, the amount the water rises is determined entirely by the volume of my body.

If Archimedes had been a sinningia grower, he would immediately have thought of tubers.  How large is that tuber?  Just put it in a bath and we'll find out!

In this case, the bath should be a graduated flask, aka a measuring cup.  Put a bunch of water in it and note the level.  Then drop the tuber in, hold it down with something like a pencil so it's entirely under water but as much of the pencil as possible isn't.  Note how much the water rose.  That tells you the volume of the tuber.

The beauty of this method is that the water molecules do all the hard stuff about figuring the irregular contours of the tuber.  No matter how ragged, jagged, and unruly the shape of the tuber is, the water handles all the calculations.

Once you know the volume of the tuber and have weighed it, you can then determine its density: weight divided by volume.  And then you thank Archimedes -- such a great guy.

Eureka, everybody!