My sister just showed me this puzzle and I cannot figure it out… Maybe I’m stupid?
"Move just one coin to make two rows of five coins each"
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i think your sister is laughting in your back…because you cannot make this…uhmm only move one to make two rows of 5…there is only one line of 4…weird…
Question… was it “two rows of 5”, as in rows vs. collumns, or “two lines of 5”, meaning any direction? Here’s my solution…
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I moved one coin from my pocket to the end of the 3rd row. Now I have two lines of 5, one vertical, one horizontal.
Think outside the box. 
Imp
Improvisar - (Spanish verb) “To Improvise”
Imp - (Websters Dictionary) “A naughty mischievous child.”
http://www.horde.org/imp/about/
just another little Imp thing for ya
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I’d make a row of six, since that gives two different rows of five 
(the lower five and the upper five)
Obviously since there’s only eight coins you must be allowed to use the same coin in both lines.
Oh, since we’re listing meanings of the word imp too 
E. Cobham Brewer. Dictionary of Phrase and Fable.
Imp (Anglo-Saxon).
A graft; whence also a child; as, You little imp. In hawking, to imp a feather is to engraft or add a new feather for a broken one. The needles employed for the purpose were called imping needles. Lord Cromwell, writing to Henry VIII., speaks of that noble imp your son.
Let us pray for the king’s most excellent majesty and for his beloved son Edward, our prince, that most angelic imp. Pathway to Prayer.
The American Heritage® Dictionary of the English Language: Fourth Edition. 2000
ETYMOLOGY: Middle English impe, scion, sprig, offspring, from Old English impa, young shoot, from impian, to graft, ultimately from Medieval Latin impotus, graft, from Greek emphutos, grafted, from emphuein, to implant : en-, in; see en–2 + phuein, to make grow; see bheu- in Appendix I.
Easy peasy lemon squeezy:
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000*
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the ‘*’ means two coins, one on top of the other. 2 rows of 5!
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Brian
Lol… I’ve got a thread that’s half devoted to my screen name. I picked this one a long time ago… probably '95-'96 because I wanted something really unique, rather than John1289312983. I did know about the implant meaning… but it really doesn’t pertain to me (unless maybe I got into the cyberpunk genre again, lol).
Imp
I don’t know what the real solution is, so maybe you guys are right with reusing so many coins.