Ask your students to imagine they are doing the Marathon des Sabres. And they get lost. They are isolated from the pack and the support crew and run out of water. After a number of hours, they are now incredibly hot and on the brink of dehydration. However, there is some respite: out of the blue, a lady with a cold 250ml glass of water has appeared and is willing to sell it to them. Ask them to think what the maximum price they would be willing to pay for that glass would be (and to note it down).
The lady then pulls out a second glass. Again, ask the students to think what the maximum price they would be willing to pay for that glass would be (and to note it down). Repeat for a third and fourth time. Thus, the students should now have 4 values.
Ask the students to plot these values on a graph (x-axis is the number of bottles of water: 1st, 2nd, 3rd, 4th; y-axis is the "maximum price I would be willing to pay").
