MAGIC BEANS
In one of his notebooks, the famous artist and scientist Leonardo da Vinci describes the following trick. (To do the trick, you need a supply of small objects such as beans.)
‘Take equal numbers of beans in each hand. Transfer four from right to left. Count the remaining beans in your right hand and cast them away. Cast away the same number from the left. Pick up five more beans. You will now have thirteen.’
In fact it doesn’t matter how many beans you started with, you will always end up with thirteen in your hands. Why?