Once you find water, a major issue remains: Is it pure? Is it drinkable? And if not, how to make it drinkable? You can make a water filler(过滤器）from a cone of birch bark(桦树皮). This cone is then filled with layers of sand, charcoal(木炭), grasses, and other materials. Grasses and sand help to trap suspended particles (悬浮颗粒). Charcoal helps to remove chemical impurities, but not bacteria. You may have to pass the water through the filter more than once, depending on the site of the filter and what it’s made of. Generally speaking, the bigger the filler, and the more layers you have in it, the better.

The birch bark cone will need to have a fairy small hole in the bottom. The cone will have to be tied with cordage to keep it from opening up. Put a few stones in the very bottom, to help hold your filtering materials in place.

Then pour in layers of charcoal, grasses, sand, and possibly other materials that you feel will help to filter out suspended particles and perhaps even bacteria.

Once you have constructed the filter, simply pour impure water through the filer, catching it in another container at the bottom.

This is basically the “well” method of obtaining water. Dig a hole about 2-4 feet from a water source, such as a river. Allow the hole to fill with water. The water in the hole will likely be reasonably pure.

Weather reports often mention high or low pressure systems headed toward a city or town. If you’re in the path of one of these systems, expect a change in weather conditions. High and low pressure systems operate using similar principles, but the effects they produce differ significantly.

View a weather map, and you may see distinct regions that represent different air masses. An air mass is a region of air that has about the same humidity(湿度)and temperature throughout. When an air mass remains over a location for a while, it decides the characteristics of the region. Air masses can be cold or warm and they may contain wet or dry air. All air masses have the ability to move away from their source region into other locations.

Meteorologists(气象学家)call the boundary between two air masses a front. A cold front’s motion through a warm front is a low-pressure system. If a cold air mass replaces a warm air mass, you have a cold front. Air in the cold air mass is often drier than air in front of it. As the air masses meet, the wind u- sually changes direction and rain or thunderstorms often occur.

A warm front is associated with a high-pressure system and occurs when a warm air mass replaces a colder air mass. A high-pressure system often brings clear, calm weather. Warm fronts travel slowly and are weaker than cold fronts.

Cold fronts move faster than warm fronts because a cold front contains denser air. Cold fronts also of- ten travel from west to east while warm fronts move from north to south. As a warm front approaches, temperatures may rise as warmer air flows in from a warm air mass. However, when the cold front hits, temperatures can drop precipitously within a short time.

Working out can help teenagers boost their grades, a new study suggests. Adolescents who routinely engaged in moderate to vigorous（剧烈的）exercise showed long-term improvements in their academic performance, the British Journal of Sports Medicine study reported.

“Our study suggests that the effect of physical activity may be quite large,” John Reilly, a professor at the University of Strathclyde said. The researchers looked at a sample of about 5000 children who were involved in a long-term study that tracks children born in the U.K. between 1991 and 1992. When children reached 11 years old, their daily physical activity levels were measured using an accelerometer（加速器）for three to seven days. The device, similar to a pedometer（计步器）tracking the number of steps taken, recorded the average time children exercised, which was 29 minutes a day for boys and 18 for girls.

“The actual levels of daily physical activity at age 11 were quite low,” Mr. Reilly noted. The children had their academic performance tested at ages 11 and 13 with compulsory national tests for students, and also at 15 or 16 with the General Certificate of Secondary Education (GCSE) exam. The tests assessed the children’s abilities in English, math and science subjects. The results showed that the more children participated in moderate to vigorous physical activity, the higher their test scores were at age 11 in all three subjects. For girls, science scores were most strongly linked to exercise.

When children were tested again at age 13, their academic scores were still linked to how much they had exercised when they were 11 years old. By the time the youngsters took the GCSE exam, each 17-minute-per‑day increase in physical activity for the boys was linked to an improvement in their scores. Every additional 12-minute increase a day in exercise for the girls was also linked to an increased score, especially in the science category.

The researchers have called for more studies to look at the possible academic benefits that could be derived if students exercised the recommended 60 minutes or more a day.