0 If a roof or ceiling falls in, it drops to the ground because it is damaged. -- (屋頂或天花板)坍塌,塌陷
1 If soldiers fall in, they form a line or move in a line, one behind the other. -- 列隊;集合
We can complete it step by step however long the road is and it can't be completed however short the road is if you don't even mark your footprint.
再长的路,一步步也能走完,再短的路,不迈开双脚也无法到达。