Some of you may find this strange, but I typically travel with diet soda.
I should say that I don't load up the car each morning on my way to work (although I do consume one can on the drive in), but whenever we are going to visit a friend or family member, I bring some soda.
What I usually do is stop by the store, pick up several bottles or 12 packs, and take one in with me when I get to the destination. That way, if they all get consumed, I know I have more in the car as backup. And there are no uncomfortable moments when I am leaving if it's not all consumed - I take the leftovers with me.
I have a few friends who always tell me: "George, I know you are coming so I always get diet for you!"
To which I reply: "Great! I will drink yours first and take this home!"
