Neper Sticks

John Neper (1550-1617)
was a Scottish mathematician, astronomer, and alchemist who invented a counting set for multiplication known as Neper's sticks. Each stick in this set contains a multiplication table by a separate number. If, for example, you need to find the number 9x371, then you should place the sticks next to the multiplication tables for the numbers 3, 7 and 1, and read the contents of the ninth row, which will look like this: 276200

There were rumors that Neper dabbled in witchcraft, having a black rooster at his disposal. He periodically ordered one of his servants to come into the room and pet the rooster, saying that the rooster would check their honesty. In fact, Neper was just smearing the bird's feathers with ashes. If the servant was "unclean", then he did not stroke the rooster, and his hands remained clean. At the same time, the cunning master immediately saw that the servant was lying.

I've made a little app that illustrates this trick. To drag elements, an API is used that allows you to assign the draggable property to elements. This allows you to abandon the usual javascript code, where to implement such an approach it was necessary to track the movement of the pressed mouse and assign the coordinates of its movement to the dragged object.