An explorer is making his way across territory that is unknown to him, and he has no map of the area. All he can do is head in the direction that he believes will get him to civilisation.
After walking for a while he comes to a river which he crosses quite easily, and then carries on in the direction that he believes to be correct.
After walking further he comes to a river which looks exactly the same as the river that he crossed earlier. Everything about this river is indistinguishable from the other river.
And yet the explorer knows with complete certainty that this is a different river - how?
Answer on 9.00 on Monday