Sharing 50 / 50

A young man saw an elderly couple sitting to lunch at Mc Donalds. He noticed that they ordered one meal and an extra cup for drink. As the young man watched them, the gentleman divided hamburger, fries and drink equally between him and his wife. Then the old man began to eat, but his wife was sitting watching with her hands folded. The young man decided to ask if they allow him to purchase another meal for them, so that they didn’t have to split, and he does so.
The gentleman said, ‘oh no’ ! We have been married for 50 years and everything has always been and always will be shared 50/50. Then the young man asked the lady why she didn’t start having? She replied, ‘ first it is his turn with the teeth/dentures !