Swimming isn't only about the technique you employ when you dive or cut through the water. Little things that make a difference. Especially at the highest level, like the Olympics. Like the type of suit you wear, the amount of body hair, the material of the suit etc.
What many fans noticed at the end of certain races at the Rio Olympics is that when swimmers like Michael Phelps, Katinka Hosszú (Hungary) and Katie Ledecky (USA) took off their caps, there was another cap underneath.
Now, this isn't like cricket where you put extra grips on your bat. In fact, many would argue that two caps might add to the weight, and on a stage where a millisecond could be the difference between glory and heartbreak, why would anyone risk it?
Well, one simple explanation is that the two caps help keep the goggles in place. Ask a swimmer, there is nothing worse than water entering the goggles. So they place the goggles on top of the inner cap and place the upper cap on top of that to hold it in place.
But the main reason is that both caps are made of different materials. The inner cap, which is made of latex, tends to wrinkle in the water and can create a drag. Therefore, a smoother surface is more important, one that does not crumple.
This is where the upper cap comes into play. It is made of silicon and provides extra smoothness, thus avoiding any drag and providing more speed. Also, because it is slippery, it can't be worn inside.
But the golden rule of swimming is that no matter what happens, keep swimming. Which was the case with US swimmer Dana Vollmer, who lost her silicon cap mid-lap but still managed to Win Gold at London 2012.
So there you have it. In a nutshell, swimmers wear two caps for two things: stability & speed!