Plumbin'
Last updated at 9:09 pm UTC on 28 January 2020
Plumbin' was created by Ward Cunningham back around 1985 in Tektronix Smalltalk as an exercise to show how much can be done with MVC, and how easily.
A modern Morphic version now exists, primarily as an example in using drag and drop within Morphic.

There is an interesting problem with the code in that having two faucets (taps to us Brits) will result in an infinite recursion as the algorithm for finding the 'bottomPressure' will reflect back through the piping, find the other faucet and reflect back down the piping to find the other faucet and ... wait, haven't we been here already?
You can load this from squeaksource
MCHttpRepository
location: 'http://www.squeaksource.com/PlumbingDemo'
user: ''
password: ''
Or you can
Installer ss project: ‘PlumbingDemo' ; package: 'Games-Plumbin-tpr.4'; install
Jump over to Drag and Drop Explained to learn about how drag and drop works.
Ward did update his original for Squeak in '97 (and added colour); this version started from his code and modified quite a lot.