One of the strangest questions in science is why dead bodies float while living bodies usually sink in water. The answer lies in biology, physics, and the changes that happen after death.
Living bodies are made mostly of water, fat, and muscle. Muscles are dense, and bones are heavy. When a person is alive, their body tends to sink because it is heavier than water. Lungs are filled with air, but unless you take a deep breath, it is usually not enough to make a person float easily.
After death, the situation changes. The body begins to decompose, and bacteria inside the intestines and tissues start breaking down proteins. This process produces gases such as carbon dioxide, methane, and hydrogen sulfide.Read The Complete Original.