What is the next entry in the series?
A, BB, CC, DD, E, F, G, H, II, JJ, K, LL, MM, NN, OO, PP, QQ, RR, SS, T, UU, VV, WW, X, Y, ______.
One thought is that it would be "ZZ".
Reason?
If you can draw the letter without taking your pen off the paper it gets written twice, otherwise only once.
Problems I have with this thought:
The following entries may or may not fit, depending on your handwriting style, or font used:
"G" - can potentially be drawn without lifting a pen
"I" - with crossbars top and bottom, this would need the pen lifting
"J" - personally, in my handwriting, it takes 2 pen strokes to make this letter.
"Q" - very dodgy this one, but kinda works in the font I'm seeing on my web browser
Hmmm - I don't know about this one. I really hope this isn't the answer as it's very poor...
Well.....
