Take a Sniff!

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Sniffing will mean to take up air inside the body through the nose. Everyone knows this because it is a usual custom now and then for humans.

Some sniff food before they eat, some just sniff the air for bad or pollution in the air. It is just a way of having a smell at something.

Then the slang definition of sniffing the air will mean sniffing out something that was not there before just like how a dog figures out a trail using its nose. In this case, you find through the details and thorough search what is hidden from others. It can be said as, "sniff it out". It is quite an interesting concept when used according to context.

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