"If it was that simple, why would experts make such a big deal out of it?".
A few examples;
1. It is the tendency of people to over complicate things and think beyond the evidence in their search for understanding.
2. Making an explanation of how something works more complicated can be a means of hiding how something works.
3. Making an explanation on how something works more complicated makes an expert conveniently necessary.
The fluid that the body is mostly made up of is not water. It is a non-newtonian fluid that has the properties of both a solid and and a liquid depending of how quickly forces are applied to it. The slower a force is applied the more like a liquid the fluid behaves like. The faster a force is applied to it the more like a solid it acts like. These forces can be external like a poke or a punch or internal.