Seals are used to keep contaminants out and important fluids in. Water is the most common contaminate partnered with dirt and sand to make mud. Most components work best when they are in a clean dry environment such as a testing lab. For example: in order for bearings to operate smoothly they require smooth balls and grease. Once dirt is introduced into this system is acts as sand paper which pre-maturely wears down the balls and races while water washes out any grease or lubricant that may have been present. This results in a rough, grinding bearing with lots of play. Seals help prevent water, dirt and sand from contaminating components but they are far from perfect. With enough force or pressure these contaminants can still get past seals. Seals are generally made from some kind of rubber or silicone and sometimes have metal inside to provide structure much like a skeleton.
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