In hot weather
Pour out a glass of icy cold water from the fridge on a hot day. Water in the air that’s been heated up and turned into vapour suddenly hits the cold surface of the glass, cools down and turns back from vapour into water droplets.
In cold weather
Breathing onto a mirror will make it mist-up. Water vapour from the lungs (where it’s very warm) hits the cold mirror, condenses and forms tiny droplets
When it's cold outside you can see your own breath condense. Air from your lungs is warm and full of water vapour. When you breathe out the cold air causes the water vapour to condense back into water droplets. This is the mist that you see.