Grease

With a droplet of that grease that make you a better flotation for  your dry flies.
Some models of dry flies must float without sinking to the first ripple. With a micro droplet of this fat you make good flotation to those who absolutely have to stay on the surface.
The vial contains a Gink from the american Brand Gehrke's, silicone grease.
Tip: never lubricate a fly still (or already) wet, as this would aggravate its immersion.