Blonde Girl With A Diving Mask In The Sea

Divers often test whether a mask is a good fit by placing it on their face, without using the straps, and gently inhaling through their nose. If the mask stays on without any help this indicates that no air is being drawn in and that the fit is good.
Use
To prevent a mask from fogging up due to condensation on the glass plate many divers spit into the mask, wipe the spit around the inside of the plate and wash it out with a little water. There are commercial products that can be used as an alternative to the saliva method.