There are several factors that contribute to why a dead body floats on the surface of water while a living body tends to sink. Let's delve into the details:

Buoyancy and Density:
- Buoyancy is the upward force exerted by a fluid on an object immersed in it. It depends on the density of the fluid and the volume of the object.
- A living body, being mostly composed of water, has a similar density to water. Hence, it tends to sink due to its weight being greater than the buoyant force acting on it.
- On the other hand, a dead body has a lower density compared to water. This is primarily due to the accumulation of gases produced during decomposition, which increases the body's volume and decreases its overall density. Therefore, the buoyant force exceeds the weight of the body, causing it to float.

Decomposition Process:
- When a body dies, it undergoes a series of decomposition processes. One of these processes is the production of gases, such as methane and hydrogen sulfide, by bacteria and other microorganisms present in the body.
- These gases are lighter than water and accumulate within the body, increasing its volume. As a result, the overall density of the body decreases, leading to its buoyancy and floating on the water surface.

Waterlogged and Decomposed Tissues:
- As decomposition progresses, the body's tissues start to break down and become waterlogged. This water content further contributes to the overall buoyancy of the body, making it more likely to float.
- Additionally, the decomposition process weakens the structural integrity of the body, causing it to lose muscle and connective tissues. This loss of mass also decreases the body's weight, further enhancing its buoyancy.

Body Position and Entrapment:
- The position of a dead body in water can also influence its floating behavior. For example, if a body is face-down, the air-filled lungs can act as a buoyant force, keeping the body in a floating position.
- In contrast, a living person will actively try to keep their airways clear by lifting their head above the water surface. This action increases the body's weight distribution below the waterline, making it more likely to sink.

In summary, the floating or sinking of a body in water is determined by factors such as buoyancy, density, decomposition processes, waterlogged tissues, body position, and entrapment. These factors collectively contribute to the observed behavior of dead bodies floating on the water surface while living bodies tend to sink.

But...when we check density of death body ..then it shows its actually weight as well as mass...and also when we checked its height or its widths... we found nothing chages ....then ???