Two paper sheet: one side red, the other white. The sheets are folded in half: one vertically and the other horizontally. Fold them one (for example, white) side out. Horizontal sheet insert in the vertical and then remove both sheets on the contrary, to horizontal out. From the vertical sheet is visible half. Now they are both red. Not pulling the vertical piece, and push it from one end to the other. He pulled away from the red turned white.
What is the secret? Piece of paper which is folded vertically in the center has a horizontal section, as shown in the figure. Inserting a sheet of paper, folded horizontally, the sheet folded vertically, skip it half through the cut out. If you now remove both sheets on the contrary, the horizontally folded piece of vertical halves will look in different directions. Therefore, the sheet and changes color, if you push him the other half.