Sometimes your mask will fog up despite having applied an anti-fog agent (or spittle), to the inside.
Solution - with one finger , gently push the mask top against your forehead. With the other hand, gently pull the lower skirt/ edge -
of the mask to let about a desertspoon of water in, then...
Facing down, move your head around to swill the water all over the mask's inner glass.
Raise your face slightly above the vertical. Now, the water is at the bottom of the mask near your
nose. Gently breathe out. Blow too hard and you get saltwater splashing into your eyes.
The mask is clear and empty of all water. With practice, this can be done without even surfacing.
The fogging can re-occur so just repeat the process. It's quick and easy.