According to Rex Boggs:
I had a hypothesis that the daily weight of my bar of soap [in grams] in my
shower wasn't a linear function, the reason being that the tiny little bar
of soap at the end of its life seemed to hang around for just about ever. I
wanted to throw it out, but I felt I shouldn't do so until it became
unusable. And that seemed to take weeks.
Also I had recently bought some digital kitchen scales and felt I needed to
use them to justify the cost. I hypothesized that the daily weight of a bar
of soap might be dependent upon surface area, and hence would be a quadratic
function ... .
The data ends at day 22. On day 23 the soap broke into two pieces and one
piece went down the plughole.