I am no english native speaker, sorry if something is a little confuse. I was swimming, and then I realized that when I let the air go our of my lungs I sink. I though this might be because of changes in the volume of my body while holding air or letting it go. So I hold as much air as I could and also make my stomach as small as I could with my muscles, and I realized that I was floating independently of the shape of my stomach, then I let air go and I sink, independently of the shape of my stomach. I did this many times to be sure. My question is, how does this is explained by buoyancy?, because it seems that the volume of my body had nothing to do if I floated or not, and I know that air is lighter than water but it only makes my heavier without expanding considerably my volume. So, how can I possibly float?