Your childhood likely involved at least one cartridge-based gaming console, and with those games came a habit that feels as natural as taking up the controller: blowing in the cartridge. It is widely known at this point that such an activity was useless, but that doesn't answer the question of why we did it.
It was a common issue: sometimes you'd stick the cartridge in and fire up the console, only for the game to load all kinds of wonky. You'd pull the cartridge out, give it a strong blow, and then put it back in -- odds are, it would work on the second try.
You probably don't remember any specific point when you were told to blow in the cartridge -- instead, someone(s) somewhere got the idea that dust collected on the contacts, making for a poor connection, and that blowing it out would make the game work. The method spread, and became commonplace.
On PBS's It's Okay To Be Smart, the phenomenon was addressed, and as you might expect, the reason we blew in the cartridges was a mixture of two things: a pattern-seeking brain and confirmation bias. Check out the video above for a complete explanation.