HOW COME? Hot tips on heat and temperature

How come heated milk sometimes develops a film on top? Also, why do metal objects feel colder than anything else in the room? asks Meghan Pinezich, asks a student in Brookville.Slimy skin lurking on the surface of your hot chocolate? Blame what happened as it cooked. Milk is mostly water, with dissolved proteins and fat. As milk is heated on the stove, its water begins to evaporate. When the temperature reaches about 120 degrees Fahrenheit, milk's proteins start to clump together with its fat, forming a distinct layer at the surface.

As milk continues to heat, this surface layer dries out, creating a thin skin that acts like a lid, keeping steam trapped below. The result can be a "boil over": A rush of steam breaks through the surface, and milk cascades over the pan's sides. To stop the skin from forming, use skim milk -- or just keep stirring.

But what about that metal saucepan you use to heat milk? Why, even on a hot summer's day, do metal pans and utensils feel cooler than wooden cutting boards and plastic spoons? Isn't everything in the room at the same temperature?

It turns out that while "temperature" and "heat" are related, they're actually two different things. Temperature measures the average amount molecules are moving in a material -- the molecules' average kinetic energy. Heat, on the other hand, is the energy a material has from the combined energy of all of its molecules. Imagine filling a small glass and a bathtub with the same warm water. While both containers of water may be at same temperature -- say, 100 degrees Fahrenheit -- the tub full of water has much more stored heat.

What does this have to do with objects feeling cooler or warmer to the touch? When we put two objects of different temperatures together, heat energy will transfer from the hotter object to the cooler.

Like that tub of hot water, the human body is a substantial reservoir of heat energy. And since heat flows from warmer to cooler objects, we are constantly transferring heat energy from our bodies to the objects (and air) of a room. (Which is why, when you stand up from a desk chair after working, the chair seat is toasty warm.)

But different materials conduct heat energy more or less easily. Metals are especially good at it; a metal pan will gain heat rapidly on the stove, and cool off more quickly than a heated glass pan.

So grab a metal pan at room temperature, and heat will flow rapidly from your warmer hand into the pan. The result: A steep, speedy drop in the temperature of your fingers. Presto: Your skin senses the pan in your left hand is cooler than, say, a wooden spoon -- a heat-conducting slowpoke -- in your right hand.