How it works
A row of towers appears, each one as tall as its number. Tap two that stand next to
each other to swap them. A tap on two that are not neighbours is refused and says why.
The Watch button runs bubble sort in front of the child, a step at a time: the pair
being looked at goes one colour, and the pair it decided to swap goes another, because
looking is not swapping and that difference is the algorithm. A fresh row is dealt
afterwards, so the answer to the solved one is worth nothing.