Cut a 10-by-10-inch piece of paper. Place the ruler diagonally across the paper so the edge of the ruler bisects two opposite corners of the paper. Draw a line along the edge of the ruler across the diagonal. Move your ruler and repeat the procedure to draw the diagonal bisecting the two remaining corners. If you have done this correctly, you will see an "X" on your paper with the corners of the "X" being the corners of your paper.
Hold your compass against the printed side of the ruler. Stretch the legs of the compass until they are 1 1/2-inches apart. Place the point of the compass (the leg without the pencil) on the center of your "X" where the two lines cross each other. Draw a circle 3 inches in diameter.
Cut along the diagonal lines of the "X" with the scissors. Start from a corner of the paper and cut along the line of the "X" until you reach the edge of the circle. Do not cut through the circle. Do this from each corner of the paper. When finished, you will have made four cuts, and you will have four triangular-shaped sections of paper.
Fold the left corner of one of the triangular-shaped sections toward the center of the paper where the lines of the "X" meet. It does not matter with which section you start. Place the tip of the corner over the center of the "X" and hold it there. Repeat with all four sections of the paper. Place a push pin through the folded corners and the center of the "X" to hold the pieces in place.
Attach your windmill to a straw. Push the pointed end of the push pin sticking out the back of your windmill through the straw about 1/2 inch from one end of the straw.
Push the pin so it goes through the straw and sticks out the other side. Place a small bit of cork or eraser on the pointed end of the push pin to hold the pin in place. Use just enough cork to go over the end of the pin.
Blow on the front of your windmill or place it in front of a fan to watch it spin.