HOW many years have you thought that the blue side of a double-coloured eraser is to rub out pen marks?
Well, you'd be mistaken - as one person found out when they took to Reddit to ask what the purpose of the two different colours is for.
"If the blue part of an eraser is not for erasing pen marks, then what is it for?" they wrote.
"The blue bit is for removing ink writing, but it always ends up with the paper getting ruined, almost like rubbing sandpaper on paper," one insisted.
"I've always thought that the blue part saves the eraser from being completely gone and you will have something firm to hold on to, to finish the job, without your fingers in the way," another wrote.
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However, stationery experts at The Pencil Case have explained the actual reason there are two separate sides to the rubbers.
"You're probably like 95 per cent of the population believing that the red bit is for pencil and the blue bit is for pen," they said.
"We have all tried in vain to rub out biro with the blue end, and ended up just destroying the paper.
"The blue end is actually designed for erasing pencil marks on heavy grades of paper (like artists' sketch pads) or really dark marks."
In contrast, the red end of the rubber should be used for "light paper and precision erasing".
Commenting on The Pencil Case's statement, one person wrote: "I came to this conclusion when I was erasing pencil marks on wood with sandpaper.
"I was like, 'Oh so this is what the abrasive side was for'."