Scratch

Hi, I'm making a puzzle game with loads of puzzles and quizes and stuff but I need help with my "tesselation" game. Here's what I mean. I have a square and you need to drag and drop all the different pieces into the square so they all fit. However, I don't know how to get to

1) Reconise when the game is won.

2) Make the pieces snap into place using a grid.

I would like to know as soon as possible.

Thanks!