How to make illegal positions

You got it right.  :)  It is just barely legal.  Black needs 10 captures to get that pawn structure.  White has lost 10 pieces (not including the promoted pawn) so every white piece needs to be captured by a black pawn.

I might make some more of these type of puzzles.  It is a different idea than trying to calculate a mate in 5 puzzle or something like that.  :)  It's not that I hate them but it is a neat idea.

Not only that, but most of black's pieces need to be captured by white pawns to move those pawns closer to the middle for black to capture them. However, black could have survived with a couple pieces left, I believe, and still achieved this position.

Eebster, your position is legal, but again, it requires very careful capturing. The two extra same-colour bishops on each side must be promoted pawns, thus no pawns must have been captured. That leaves 6 pieces that can be captured (as there are 7 pieces but you must keep the bishop). The pawns only need to make 5 captures in order to reach that arrangement; but the tricky part is realizing how to get two passed pawns for each side. That can be solved by using an "asymmetrical" setup that gives both sides a passed pawn each, then using the last capturable piece to give each side the other passed pawn they need...

Oh, another interesting subtlety of the position was having to watch out for the colour of the promotion square...I originally tried a "symmetrical" setup using the a to f pawns to create the final formation, while promoting the g and h pawns - this method also allows using only one capture per side to get two passed pawns each (e.g. bring both black pawns to the g-file and both white pawns to the h-file). But I realized that didn't work because the promotion squares for both sides would be the same colour...so I was stuck for a while until I realized an "asymmetrical" setup would also give each side a passed pawn. It also took a while to notice that once the 7th-rank bishop is in place the king can't get into position unless it's already in place...come to think of it, I just realized this is a stalemate position.

EDIT: Solution has been changed to lose a tempo with the rook and make it White to move at the end.

Legal or illegal?  :)  I think this one is a good one.

 The only problem with that position is...Black to move. I'm sure that could easily be changed by losing a tempo with the Black bishop.

Yeah, either side can give a move from that position.

That's interesting, I thought I might have miscounted. The follwing position is illegal. Can you tell why?

 ur second one isn't legal either. white could have played the bishop out and then the kiing, then the queen. then moved them back in again

Good point. It's probably easier to lose a tempo with the rooks towards the end though...post has been edited.

You are right.  :)  I actually tried to setup an illegal position, but I overlooked one thing.  You showed that it was legal.  I made a couple changes to the diagram.  Unless I overlooked something else, I believe that this diagram here is illegal.

Here's the solution to  SerbianChessStar's first position:

how about THIS? legal?

It is illegal since both kings are in check.  :)

The only legal position that can lead to that position is with the black king at h2, in check, because there is nowhere the knight could have come from and the king could not have been adjacent to white's king. One tempo before that, white must have not had black in check (since it was white to move), but there is nowhere the rook could have come from, so the check must have been discovered. Without white's knight, there would be nothing to block the rook check, and the position would have been illegal.

With the knight, legal:

Without the knight, still legal:

EDIT: Oh wait, you mean the WHITE knight.

I fail to see how that is illegal. White captured at least five of black's pieces with his pawns, but there are more than five black pieces missing, and nothing needs to be the result of a promotion. Other than that, nothing really jumps out at me other than the odd position of white's king, but it doesn't look difficult to move it there.