Draw ten lines on the notebook sheet, as shown in the photo.
Draw another line that will pass through the 8th line, and the extreme only barely touch.
Cut the sheet along the last line and fold it as shown in the photo. The lines have recovered, but their number has decreased!
Where did the 10th line go? Not everyone can answer this question.
However, everything is very simple. After all, the lines have increased in length. And the length of 9 new lines equals the length of the first 10 lines.
The 1st line completely joined the segment of the 2nd. Most of the 2nd line joined the third segment, and so on. Accordingly, the small rest of the 9th line joined the whole 10th. And no magic!