I've been thinking on this one. A bottle of wine in a regular mid- to high-end restaurant requires very little work - the glasses are already on the table, all they have to do it fetch the bottle, open it and pour. So the diner had 2 bottles of wine. Had the table been drinking e.g. Coke, the server would have had to place the order in the bar, collect drinks, bring them to the table, and then check for and replenish drinks as necessary. In most instances, people have refills on soft drinks (they may drink more wine, but that is chargable), so the workload for a Coke is greater than the wine. The Coke may cost $3, unlimited refills, the wine more like $60 for an inexpensive bottle at a mid- or high-end restaurant. Why would you tip $12 / bottle for the wine ($24), but only 60 cents for the one that requires more work? The more I think on this, the more drink tipping doesn't make sense. Maybe we should tip on the food, but accept that a $300 bottle of wine takes a similar amount of effort to the $3 Coke, and just not tip on the drink portion of a restaurant bill. Certainly in a bar, I was always told by US friends, $1 a drink is what you tip - maybe apply that if you are not comfortable with no tip.