The Straight Dope 

Why do so many public buildings want you to use the revolving doors rather than the regular doors? --Seamus McCafferty, Hoboken, New Jersey

As with many things, there are two reasons--the ostensible reason and the real reason. The ostensible reason is that the revolving doors create an air trap. Since the interior of the building is never directly exposed to the outdoors, there's less chance of all that expensively heated or cooled air getting out and running up your utility bill. An alternative way of accomplishing the same thing is a vestibule, where you have to pass through two sets of doors to get inside. Another reason for revolving doors is to prevent wind from howling down (or up) the elevator shafts and stairwells and blasting out (or in) the doors due to indoor-outdoor pressure differentials.

OK, so much for the cheesy rationalizations. The real reason you're told to use the revolving doors is so the real estate operators of the world can test your willingness to play ball. Do you follow directions and use the revolving door, or are you one of those independent types who insist on doing their own thing? If the latter, be forewarned: when Donald Trump takes over, you're history.

Why is the word "AMBULANCE" typically stenciled backwards on the front of an ambulance? --C.L., Washington, D.C.

Uh, C.... you wouldn't happen to work for Vice President Quayle's office, would you? As anybody with two operating brain cells has already figured out, the lettering is reversed so it'll appear correctly in the rear-view mirror of any motorist in front of the ambulance.

What is the significance of June 19, better known as "Juneteenth," in black American history? It's celebrated as a holiday and presumably marks some kind of emancipation, but I can't find out any more. --Marianne Dornbush, Goleta, California

Juneteenth, also known as Emancipation Day, is observed in parts of Texas in honor of the June 19, 1865, decree freeing Texas slaves. Sorry if this mention is a little overdue, but let's face it, so was emancipation.

Long ago I noticed that the bubbles in clear carbonated beverages seem to stream from fixed spots on the bottom and sides of the glass containing them. Boiling water seems to behave in a similar way. What's so special about these spots? --David Peterson, Washington, D.C.

Seems to me we talked about this recently, but what the hell, it's one of those subjects worth coming back to again and again. The places from which the bubbles stream are known as nucleation sites. They're microscopic defects or bits of crud on the glass. When water is changing phase (e.g., boiling, condensing, freezing), it needs a place where the vapor bubbles, droplets, crystals, or whatever can glom together for a while until they're big enough to survive. That's what nucleation sites provide. Snowflakes and raindrops, for instance, typically form around dust motes. When water reaches boiling point, the scratches in the container provide hideouts where microscopic bubbles can collect long enough to become big bubbles.

The carbon dioxide bubbles in beer and soft drinks work the same way. Before you uncap the bottle, the pressure inside keeps all the CO2 in solution. After uncorking, the reduced pressure enables the gas to slowly boil away, which is where the nucleation sites come in handy. If you want to see some serious bubble action, try sprinkling salt in your beer. The salt provides numerous nucleation sites, producing not only a fascinating demonstration of physics, but heaps o' fun besides.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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