The brain has a density of about 1.05 grams/cubic centimeters... who cares, right? What's so special about the brain? Now, with the exception of the connection between the brain and the spinal cord, the brain is solely suspended in the cerebrospinal fluid (CSF), which has a density of about 0.997 grams/cubic centimeter. CSF acts as a cushion for the brain and allows it to suspend in the skull.
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The issue with the CSF is that it is the only protection of the brain, which makes it extremely easy for the brain to move around within the skull. When the brain gets shook within the skull and hits the bone of the skull (for instance, getting hit in the head, falling on your head, etc.), concussions can occur. This is why in all sports that use very hard balls and heavy contact require proper helmets or some form of head/eye/mouth protection.
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Watch the short clip below to learn more about concussions.