The pattern which has been introduced in Lesson 12-4 is also used to indicate that the actions in pairs, represented by V1 and V2, occur alternately.
この にんぎょうは 目を 開けたり 閉じたり します。
This doll [repeatedly] opens its eyes, and closes them.
この 電気は ついたり きえたり します。
This light goes on and off.
あの 赤ちゃんは ないたり わらったり して います。
That baby over there is crying one minute, and smiling the next.