Suppose that Keltham is indifferent between 3 black jellychips and 4 blue jellychips, and that Limyar is indifferent between 2 blue jellychips and 3 black jellychips.  Suppose they both start with 12 black and 12 blue jellychips.

Then for Keltham to trade his 12 blue jellychips, for 10 black jellychips from Limyar, would leave them both better off.

And for Limyar to trade his 12 black jellychips, for 9 blue jellychips from Keltham, would leave them both better off.

And for Keltham to trade his 12 black jellychips for Limyar's 12 blue jellychips would leave them both better off.

All three of these are mutually beneficial trades.

But which of them is fair?  Or fairest?

If you're the sort who agrees to just any trade that's mutually beneficial - like Limyar, in this classroom, had been earlier arguing people ought to do - then you know what Keltham is going to do to you?

That's right.  Keltham is going to offer 9 blue jellychips for your 12 black jellychips, you're going to accept, Keltham is going to carry out the trade, and then Keltham is going to angrily throw another 3 blue jellychips at you and yell that you're being stupid.

Suppose that Keltham is indifferent between 3 black jellychips and 4 blue jellychips, and that Limyar is indifferent between 2 blue jellychips and 3 black jellychips.  Suppose they both start with 12 black and 12 blue jellychips.

Then for Keltham to trade his 12 blue jellychips, for 10 black jellychips from Limyar, would leave them both better off.

And for Limyar to trade his 12 black jellychips, for 9 blue jellychips from Keltham, would leave them both better off.

And for Keltham to trade his 12 blue jellychips for Limyar's 12 black jellychips would leave them both better off.

All three of these are mutually beneficial trades.

But which of them is fair?  Or fairest?

If you're the sort who agrees to just any trade that's mutually beneficial - like Limyar, in this classroom, had been earlier arguing people ought to do - then you know what Keltham is going to do to you?

That's right.  Keltham is going to offer 9 blue jellychips for your 12 black jellychips, you're going to accept, Keltham is going to carry out the trade, and then Keltham is going to angrily throw another 3 blue jellychips at you and yell that you're being stupid.