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I like fun fact like that. Reminds me of mental floss on Facebook.

Check out this guy he is also a talented photogropher on steemit https://steemit.com/photography/@joelinux/we-have-a-talented-steemit-photographer

I always assumed they were there for texture so the cup was a little less slick in your hand.

Edit: I didn't have the images on and I was picturing the ones with all the lines in the middle part. Ignore me.