It's a simple math exercise...
The sum of eight and five is thirteen.
To get that answer, you need to put three into the "units" column, and carry one into the "tens" column.
You need the remedial math to understand my (admittedly lame) story.
I was nine.
The guy that owned the local hardware store was a personal acquaintance. He drove a truck, and his dog ran alongside him, following him everywhere he went.
One day the dog somehow got beneath the wheels of the truck and his right rear leg was crushed so badly the Vet had to amputate it.
In an amazingly short period of time the dog learned to function almost normally without that leg, once again running alongside the hardware store owner's truck.
We named that dog "Thirteen".
It was an expert at "putting down three and carrying one".
And in spite of his name, we actually thought he was pretty lucky.