我们来做两个环:剪两张较长的纸条,在纸条两端相应位置依次标上字母A、B、C、D(图1),将其中一张纸条直接粘合两端(即点A、B重合,点C、D重合),得到一个普通环(图2);将另一张纸条先扭半圈,即扭转180°(图3),再粘合两端(即点A、C重合,点B、D重合),得到一个环(图4),人们把这个环称 Let’s make two loops: Cut two long strips of paper, and place the letters A, B, C, and D at the corresponding positions on the two ends of the strip (Figure 1). Bind one of the strips directly to both ends of the strip. (ie point A, B coincide, points C, D coincide) to get a common ring (Fig. 2); twist another note first half a turn, that is twist 180° (Fig. 3), and then bond both ends ( That is, points A and C coincide, points B and D coincide) and a ring is obtained (Figure 4). People call this ring