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There are two special keys

Have you ever noticed that the F and J keys have a small bump?

If you look at the keyboard you may not see that mark, but if you slide your finger over F and J you will feel the keys are not completely smooth.

Find the letter D and the letter F on the keyboard and gently slide your finger over them — do you feel the difference?

Curious, isn't it? You can do the same test with the letter H and the letter J.