Someone put this conundrum on Facebook recently (you have to determine which tank(s) are full first) and it brought an old joke to mind.
A chap was walking along a canal and spotted someone on the other side fall in and start to flail around in the water. The poor chap obviously couldn't swim and was in danger of drowning. Suddenly another chap dived into the canal and pulled the drowning chap to the side, dragged him half out of the water, such that his lower half was still in the canal, and started to administer Holger Nielsen resuscitation (an old technique that's no longer used). Well, water came pouring out of the half drowned chap's mouth, and kept pouring out, and kept pouring out. The first chap who had witnessed the whole event looked a bit pensive and then called out to the rescuer; "Excuse me. I'm no hydrodynamicist, but it strikes me that unless you get that chap's backside out of the water you're going to pump the canal dry."
Ref yesterday's paradox: could it be that May is waiting it out for enough Labour voters to drain to UKIP (as paradoxical as it may seem) so the 17m Brexit voters are evenly split, with neither Labour nor UKIP being a large enough threat in their own right, and then calling a snap election on a Remain platform and calling for the LibDems to join her in a dash for first past the post?