Last night, when the pizza was delivered, I made my boyfriend run out and pay for it. Correction, I paid for it---hey, I'm an equal opportunity kind of girl---but he signed the credit card slip. The reason? Tipping makes me nervous. Or rather, knowing how much to tip makes me nervous. And it's even worse when the delivery guy is standing right there. Math in a bar? Taxing, but doable. Pressure math in the lobby of your building? Much scarier.

Of course, tipping is fraught with even more uncertainty when you're in a foreign country. How much to give? Is it included? Can you add it to the credit card bill or should you leave it on the table?

If you've been wondering how much extra moolah you're going to want to cough up on vacation, here are a few guidelines.

In England, a service charge of 12.5 percent is usually included at restaurants (if not, you can leave it on the table yourself, if you like.) When taking a taxi, a ten percent tip is generally expected; depending on the fare, you might just want to round up to the next whole number.

In Mexico, 15 percent is a fairly decent tip for good service at a restaurant. In general, taxi fares that have been negotiated beforehand will usually include the tip---or at least the driver's idea of what he'd like as a gratuity---though there's certainly no harm in handing over a few extra coins if you feel you got a particularly good deal in the bargaining.

In France, a service charge is usually included in the bill at the end of your meal; if not, 15 percent is pretty standard. Taxi drivers generally don't expect to be tipped.

In Russia, it's good practice to tip 10 percent at restaurants, and between 5 and 10 percent for taxi rides.

In China, restaurants don't---by and large--expect a tip. I've heard of people being chased down the street by waiters worried that they'd forgotten to take some of their money with them. Of course, that doesn't mean you can't tip if you really want to, especially if the service was particularly good.

For all the other countries of the world, there's a great tipping guide here. And you'll find some general rules for leaving a gratuity in a foreign country here. For situations other than eating---taking a guided tour, for instance---the amount you tip (if any) is really left to your own discretion. Do remember that in many poor countries, a little goes a very long way.