Photo by Rick Geary

The helium starts seeping out the minute a latex balloon is inflated.

Dear Matthew Alice: A couple of weeks ago, I bowed under pressure at the supermarket and bought my three-year-old daughter one of those helium-filled Mylar balloons. Now as I write, I’m staring at this thing and wondering why it's still flying, even after it has become limp from loss of gas. In contrast, helium-filled rubber balloons rarely last a night before they're on the ground, even without such an obvious loss of inflation. What is it that makes Mylar balloons so much more lofty, oh mighty meister of miscellanea? — Bob in San Diego

Glad to see the balloon is still amusing someone. If your daughter is typical, she’d lost interest in it by the time you got the groceries in the car.

After checking with some local balloon brains, I predict that little Mylar goodie your sweetly innocent-looking extortionist weaseled out of you still has another week’s flying time. The reason’s not much of a mystery, though. The latex in a rubber balloon is actually very porous. The helium starts seeping out the minute it’s inflated. Because it tends to maintain its inflated shape, the latex balloon may not look like it’s lost much helium, but it has, enough to ground it after about 12 hours. Mylar, on the other hand, is solid. The only leaky point is the closure, so it loses gas much more slowly. Mylar develops folds when it deflates, making the loss of gas easier to see. See? Si.