(I previously asked this at The Mathematics Teaching Community, but I'm hoping it would attract further answers here.)
The Wikipedia page on proportional reasoning mentions a "water triangle" task. It seems that the task was created in the 1970s, possibly by Robert Karplus. However, I was not able to find any reference to it. Does anyone know of any publication that mentions its source?
Consider a container of colored liquid inside a right triangle where the triangle can be tilted and the water levels on the left and right side can be measured on a built-in scale. This is called a "water triangle":
The water triangle is rotated until it shows a measurement of 4 units on the left side and 6 units on the right side. Suppose the triangle is tilted even more until the water level on the right side is at 8 units. Predict what the water level in units will be on the left side.