You have two people in the center of an endless plane.

Each one is holding some item that can fit in their palm, like a tennis ball. Both persons are identical in physical ability and brainpower.

However, each one is just as clever as sly as the other one, and both are very clever and sly in their own right. They will immediately do what is best for them, and will always do what is best for them, unless there is an incentive.

Q: What foolproof way is there for these two to exchange their items?

Examples of inappropriate ways:

Throw it over each other: One might not throw it and chase the others item

Each hold the others item at the same time: One might let go first and the other won't let go, taking both items


Both people set their items down a ways apart, then simultaneous walk toward each other's item.

Neither person can benefit from being sneaky by returning to pick up her own item, as the other person would then do the same. There is no way either person can end up with both items.

  • $\begingroup$ I feel like this is on track, but can you define "a ways apart" for me please? $\endgroup$ – hLk Dec 3 '14 at 1:51
  • $\begingroup$ I just mean some distance apart, it doesn't really matter how far so long as as the items aren't put in the same place $\endgroup$ – Julian Rosen Dec 3 '14 at 1:53
  • $\begingroup$ Oh I see, so they can both put their items down say ten feet apart, and then walk toward each others items, crossing each other and keeping an eye on each other the whole time. Very clever! $\endgroup$ – hLk Dec 3 '14 at 2:00
  • 1
    $\begingroup$ Reminds me of the Diablo II trading experiences with things like Gheed's Charm ^^;. Had to do ground trading like that. $\endgroup$ – Xrylite Dec 3 '14 at 18:24

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