That's a sound argument

As part of our Thanksgiving traditions, we went apple picking at a nearby orchard. Before arriving at the apple farm, we had to make a stop at the bank for some cash. Devlin, who was eager to reach the destination, began to whine about the unexpected, albeit short, delay.
Devlin: Why do we have to get money?
Mommy: To pay for the apples we pick.
Devlin: Why?
Mommy (quite exasperated): Well, we've tried to sell you, but no one is interested. And they won't accept you as currency so we need money.
Devlin: You can't sell me! (in an insulted tone)
Mommy: Why not?
Devlin: Because I'm your son.
Daddy: Is that the best you can do for an argument? You'll never make a great lawyer if that's your argument.
There was some silence from the back of the van, and then this...
Devlin: You can't sell me because I'm your son and I'm going to take care of you when you get old.
Daddy: Now that's a good argument!