I often use a logic puzzle as an ice-breaker to get workshop participants thinking and working together. Working under time pressure often also brings out some interesting group dynamics.
Problem
A glass of gin and a glass of tonic stand side by side. Take a measure (eg, a thimble) from the gin glass and empty it completely into the tonic glass. Then fill the measure again from the tonic glass and pour it out completely into the gin glass. Does the gin contain more tonic than the tonic contains gin?
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Solution
A lot of people guess ‘yes’ since, intuitively (?), when we fill the measure the second time, some of what gets into the measure is a very small fraction of the gin we put into the tonic glass in the first place. So it looks like the gin glass is getting less tonic put back into it. But, by the same token, less tonic is being taken out of the tonic glass in the first place.
In fact the contamination is identical in the glasses. The simple logic is: Whatever gin isn’t in the gin glass must be in the tonic glass. This extra volume must be compensated by tonic no longer in the tonic glass, and the only place that can be is in the gin glass! And the answer is the same irrespective of how well the liquids are mixed after the first transfer!
Phrased slightly differently, here is my favourite answer for getting past peoples’ resistance to the simple approach…
[You Say] “Where is the missing Gin?”
[They Say] “It's in the Tonic glass, dummy!”
[You Say] “How much Tonic was displaced to accommodate the Gin?”
[They Say] “Er, the same amount, duh!”
[You Say] “Where is it now?”
[They Say] “Blast!”
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