Why do the ice cubes in my freezer often develop stalagmitelike spikes?
Stephen Morris, a professor of experimental nonlinear physics at the University of Toronto, explains:
Water is one of those rare materials that expand while they freeze. If a crust of ice with a small hole in it forms over liquid water, the crust can trap the liquid below, leaving it no room to expand. So as the water begins to solidify, it is forced up through the hole and starts to freeze around the edge, forming a hollow, water-filled “ice spike.
Water keeps moving up the spike—creating a little self-made chimney, which can grow quite long and thin. Eventually all the water freezes and the spike becomes solid. The energy needed to lift the water up into the spike comes entirely from its expansion during freezing.
Some features that govern the way water crystallizes play important roles in the spike formation process. The form of the ice crystals depends on the cooling rate and therefore on the air temperature. Spikes typically form when the ambient air is well below zero degrees Celsius, allowing for rapid freezing—an air temperature of about –7 degrees C (roughly 20 degrees Fahrenheit) turns out to be optimal.
Fast cooling favors the formation of sheetlike crystals, which quickly cover the surface. Some sheets hang down into the water like curtains; these crystallites tend to join to one another at 60-degree angles, much like the arms of a snowflake do. They typically leave triangular holes in the surface; hence, spikes often have a triangular base. The sides of the spike are sometimes a continuation of preexisting crystallites below the surface of the freezing water, which explains why some spikes can extend from the surface at steep angles. It is also possible to get structures that look like little inverted pyramids, or “ice vases,” if the water happens to drain out before completely freezing.
Pure water works best for spike formation, as does a container with vertical sides, such as the ice tray in your freezer. People often see spikes in birdbaths or pet drinking dishes that are left outside overnight, and they can sometimes form on falling sleet pellets. But it is rather rare for them to form elsewhere in nature—on the surface of lakes or ponds, for instance—because usually the cooling rate of natural bodies of water is not fast enough.
Does sleeping after a meal lead to weight gain?
Jeremy Barnes, a professor of health promotion at Southeast Missouri State University, offers this answer:
Although it is difficult to provide a definitive answer without knowing all the aspects of an individual’s lifestyle and genetics, it is true that going for a brisk walk rather than, say, taking an afternoon nap will burn more calories.
Sleeping itself, however, is not the cause of weight gain. The real key to weight control is energy balance (the balance between calorie, or energy, intake and expenditure) over extended periods. When energy intake is greater than energy expenditure, the body will store excess energy as fat, regardless of whether those extra calories came from fat, carbohydrate, protein or even alcohol. Unfortunately, in the U.S. many people consume more energy than their body is using, which has led to a situation of epidemic proportions where about one third of all adults are now obese and another third are overweight.
Interestingly, a few recent studies indicate that individuals who suffer from sleep deprivation or get only limited amounts of sleep actually may be more susceptible to weight gain than those who get adequate sleep. But this phenomenon seems to be attributable primarily to sleep-triggered changes in the levels of hormones that increase or decrease hunger rather than the amount of sleep itself.