HOW COME? Chemical reactions; hummingbirds

How come diet soda and Mentos candy react with each other? Also, how can hummingbirds flap their wings so fast? asks Daniel McGuirk, a student in Brookville, NY.

Drink a carbonated soda, and you may feel tiny bubbles flying up your nose. The fizzing bubbles are actually tiny parcels of carbon dioxide gas, pumped into the soda under high pressure. Open the can or bottle, and there's a hissing rush of CO2 molecules, making their great escape.

You can accelerate the process by dropping ice into your glass of soda. Ice cubes provide more handy nucleation sites, rough spots where bubbles attach and grow. Presto: Thousands of gas bubbles rising swiftly to the surface, causing a soda waterfall over the sides of your glass.

While both sugar-filled and diet sodas release more CO2 when ice is added, diet drinks are often extra-foamy. According to chemists, some artificial sweeteners, like aspartame, act like surfactants. (Soap and detergent are also surfactants.) Surfactants reduce water's surface tension, help create and stabilize foam (like the soapy foam in an agitating washer.)

But to make the most impressive soda foam, try adding a few pieces of Mentos. The roughness of the candies' surfaces (visible at the microscopic level) provide plenty of nucleation sites for bubbles. Meanwhile, like aspartame, the candy's gums and gelatins reduce water's surface tension, allowing CO2 bubbles to expand more easily. The result: Bubbles form frantically on the submerged mints. And soda shoots out of a soda bottle or glass in a sticky geyser.

As for equally frantic hummingbirds, biologists say the tiny birds can beat their wings so quickly mainly because of their body size and wing length. (A ruby-throated hummingbird measures only about 3.5 inches long.) In general, the bigger and longer the body part, the slower it moves.

Pretend your unbent index finger is a wing, and flap it up and down as you watch a clock tick off the seconds. Now, try flapping your arm. While your finger can flap speedily, your arm is a relative slowpoke. But a hummingbird puts your finger-flapping to shame, its tiny wings beating about 18 to 80 times a second, in a blur of motion.

A hummingbird's rapid-fire movements burn extraordinary amounts of energy. When a hummingbird is sitting quietly on a branch, its heart still beats about 550 times a minute. And when the tiny bird is engaged in aerial acrobatics, its heart can speed up to over 1,000 beats a minute.

An average-sized person whose body burned energy at the rate of a hummingbird's would have to eat about 155,000 calories a day.