Here's a hint... Tarkatower is believed to have made this puzzle. 

1. b8=Black Knight. simplest thing ever!

Holy cow... I can't believe you got that...

Yeah, here's the story.. a long long time ago in a galaxy far far away.. tarkatower noticed that the rules stated a pawn on the 1st or 8th file must promote.. it didn't say which colour! 

benws wrote: 1. b8=Black Knight. simplest thing ever!

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 Then king takes the knight

LOL, but it cant turn to black D:

Oh u mean black knight

it could before..

they changed the rule 'recently'. 

Wow! I had no idea about that! But you said that they changed the rule. Does that mean that the pawns "today" has to promote into the same colour?

I believe so... chess.com and every other site I've played on makes me.

I was going to say that but you got to it sooner... I've seen this in Chess Magazines

 

it really is awesome and should actually be kept that way 

i cant see any knight here!!

I'm so confused, how is it check mate on the white king???

can someone explain 

white promotes pawn to a black knight (rules have changed since then and so it is now illegal to promote to opposite colored pieces)

nice clue, briliant puzzle!

does anyone know when they changed that?

redhotman wrote:

I'm so confused, how is it check mate on the white king???

can someone explain 

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Its mate for black, not white. White promotes to a black knight, putting the black king in check by the rook. Black king can't go anywhere, and the reason for the pawn promoting to knight is that a knight doesn't do any good for black here...so its mate.

April fool's day is today isn't it?

okay.

would this also work?

No it wouldnt. 1...Kxb8. This is a nice puzzle though. I didn't even know about that old rule.