A cold front is a warm-cold air boundary with the colder air replacing the warmer. As a cold front moves into an area, the heavier cool air pushes under the lighter warm air that it is replacing. The warm air becomes cooler as it rises and, if the rising air is humid enough, the water vapor it contains will condense into clouds and precipitation may fall.

An ecosystem is a biological community of interacting organisms and their physical environment. This includes both the biotic (living) and abiotic (non-living).

If no oxygen is present, cellular respiration is anaerobic and will result in fermentation where either lactic acid or alcohol is used instead of oxygen.

The rate of vibration of sound is called frequency and is measured in hertz (Hz). One hertz is one repetition per second and sounds with high frequency have a higher pitch than sounds with lower frequency. Humans can hear sounds in the range of 20 Hz to 20 kHz.

Found in both animal sources (meat, fish, eggs, cheese) and vegetables (beans, nuts, some grains), proteins are important for the body's maintenance, growth, and repair.