My visualization of a successful contractual arrangement as interacting gears came from a toy I had when I was just a toddler. I would play with that toy for hours; little did I know then that I was creating “toddler contracts,” to my way of thinking.
The toy consisted of a flat wooden board of about twelve by eighteen inches with evenly spaced holes, a number of short wooden pegs of equal length and a number of flat wooden cogs with diameters that varied between one inch and three inches. One of the cogs had a small handle that stood upright from the flat cog. The objective of the toy was to try to arrange all of the pegs and cogs on the board so that if placed properly, the turning of the one cog with the handle would smoothly engage and rotate all of the cogs at the same time.
To my enjoyment, I could turn the whole world (of the board) with one handle! If the cogs were not arranged properly, however, the gears would lock up and not turn and I would have to engage in a series of trial and error rearrangements to get it right. Trial and error and patience were rewarded by making all of cogs turn together smoothly, a very satisfying result. The cogs could be rearranged over and over again in many different ways and figuring out how to get the whole world of the board to turn at once would keep me busy for hours (much to my parents delight, no doubt!).
(I do not believe the toy I remember is available anymore, but it is similar in concept to this one: Turn & Learn Magnetic Gears, although the toy I had was horizontal, not vertical.)