Without the tunnel mouth being properly set up a derailed train will bounce off an invisible barrier when it tries to pass under the ground. 如果沒(méi)有適當的設置隧道口，一列出軌的列車(chē)將在進(jìn)入隧道試圖穿過(guò)地面進(jìn)入地下時(shí)被看不見(jiàn)的柵欄彈回。
He bounced ideas off colleagues everywhere he went. 他在同事中逢人便試探地大講他的想法。
If I did try to do anything, I'd only get sent off with a flea in my ear. 假使我真的去試探一下，準會(huì )碰一鼻子灰。
