Guess who's coming to dinner? Meet your surprise holiday guest: Physics!

The last thing on your mind when you sink your teeth into a perfectly roasted drumstick may he science. "But when you start asking questions about the world, you'll find science everywhere," says physicist Lou Bloomfield. fine of the best places to look is right on your dinner plate.

(1) Inspector Gadget

This is your job: As dangling poultry carcasses glide past on a production line, you turkey trot alongside them, inspecting 35 birds a minute. You have two seconds to spy any visible defect or disease, which would make the bird unfit to eat. After an eight-hour shift, your glazed eyes have pored over 16,800 dead chickens! Having fun yet?

Engineer Yud Ren Chen doesn't think so. That's why his team at the Agricultural Research Service in Beltsville, Md., whipped up an invention to lighten a poultry inspector's load. The robotic inspection system is called "machine vision," and "sees" with two digital cameras, a light probe, and a spectrophotometer (device that measures light intensity at different wavelengths). The gadget scans 140 birds a minute and instantly feeds data into a computer.

One camera shoots the bird's front; the other, its back. Like human eyes, each camera detects light waves, rays of energy that travel through space and matter (see diagram, above). But while human eyes can miss subtle color abnormalities on a bird's skin, the cameras catch it all: "We designed special light filters for the lenses to reveal features like bruises, tumors, or bloody flesh," Chen says.

Robots may command superior vision, but humans ultimately determine which birds are edible. When the computer spies a defective carcass, a red light flashes and a "live" inspector reexamines the bird.

Machine vision is slated for testing in poultry plants this month.

(2) Sop Science

Leftover gravy isn't the world's most pressing problem, but it is common: 150,000 gallons per week are trashed in England alone, says Len Fisher, author of the book How to Dunk a Doughnut. He deployed the scientific method to find out which foods absorb the most gravy. By plunging meat, veggies, and breads into gravy, then removing and weighing each item at time intervals, Fisher hit on a gravy equation:

PERCENT GRAVY UPTAKE = (W-(D/S))/D x 100

W = uncooked weight of the food

D = cooked weight of the food difference

S = shrinkage, or the weight d' between cooked and uncooked food

Using his equation, Fisher found that ciabatta, an airy Italian bread, boasted a whopping 120 percent gravy uptake, almost double the absorption of plain white bread. What makes ciabatta a super sponge? The unique way microscopic particles hold the bread together. Basically, "there's a strong chemical attraction between the bread and gravy," explains University of Virginia physicist Lou Bloomfield.

Consider gravy. It's up to 90 percent water, a molecule with two atoms of hydrogen and one atom of oxygen. The ends of the molecule are polar: like a battery, the hydrogen end carries a positive charge; the oxygen end, negative. Since opposite charges attract, water's negative poles grab the positive poles of other molecules, like hydrogen in carbohydrates such as breads. Carbohydrates are nutrients made of long chains of carbon, oxygen, and hydrogen molecules, explains Bloomfield.

And ciabatta, unlike your basic white bread, is riddled with cavernous air holes, which help funnel more gravy into the bread and leave less on your plate!

(3) Ketchup Trouble

Do you like ketchup on everything--including the holiday roast? Then you know the trouble with this tasty condiment. One second it won't flow, the next it gushes like lava.

Welcome to the unusual world of pseudoplastic liquids, like ketchup, whipped cream, and toothpaste. The viscosity--or thickness--of these liquids depends on the amount of stress they're subjected to. "Ketchup behaves like a solid until enough force acts on it," says Massachusetts Institute of Technology mechanical engineer Gareth McKinley. "Then it flows as a liquid."

How much force--pushing or pulling action--does it take to move ketchup? The answer depends on its yield stress, the minimum amount of stress needed to change the liquid's shape. Give the bottle a whack greater than ketchup's yield stress, and get out of the way! "Nothing is happening, then all of a sudden it moves too fast," explains NASA physicist Greg Zimmerli.

The reason for such behavior? Ketchup is made of many ingredients stirred together. In a bottle, the ground tomatoes form a gel that thickens over time. The best way to crack the gel: "A few gentle whacks on the back or neck of the bottle and a little patience," says Zimmerli. Or, suggests McKinley, try a squeeze bottle!

(4) Nuts for Physics

A bowl of mixed nuts may be prime holiday snack food, but for physicist Paul Quinn it's a nutty physics project. Quinn was puzzled by an odd nut-bowl phenomenon: Chunky Brazil nuts always seem to sit on the top of smaller peanuts, almonds, and cashews. But shouldn't gravity's downward force pull the heavy nuts--those with more matter, or mass--to the bowl's bottom, while lighter nuts rest on top? Quinn calls the phenomenon, the Brazil-nut problem, or BNP. And the shuffling can occur in any granular mixture--from trail mix to cement.