You can imagine a bead with a wire going through it and the bead can only slide back and forth.(你可以想象一個有金屬絲穿過的珠子而且這個珠子只能前后滑動。)

He continued walking back and forth, muttering to himself; he had forgotten the King entirely.(他不斷地走來走去，喃喃自語；他完全忘記了國王。)

The pendulum swings back and forth.(這就像單擺一樣來回擺動。)

The prosecution and defence were bandying accusations back and forth.(原告與被告你來我往，互相指責。)

If two flying insect populations took up residence in separate nearby valleys, chances are that individuals from each population would fly back and forth, continuing gene flow.(如果有兩個飛行昆蟲種群居住在相距不遠的不同山谷中，那么任一種群中的個體都有可能來回飛行，從而使基因流動繼續。)

The people swayed back and forth with arms linked.(人們手拉手前后搖擺著。)

She passed the needle through the rough cloth, back and forth.(她用針在那塊粗布上來來回回地穿進穿出。)

They continue calling back and forth until they all find each other again.(他們不停地來回打電話，直到他們再次找到彼此。)

From what I remember, water changes back and forth from water in lakes and oceans to vapor, and then back to water again when it falls as rain or snow, as precipitation.(根據我的記憶，水往復地變化，從湖泊和海洋里的水變成水蒸氣，然后當它以雨或雪——也就是降水的形式落下時又變成水。)

So energy can go back and forth.(所以能量可以來來往往。)