Idioms based on architectural and engineering structures. Select an idiom for more details. Also try Structures Idioms Quiz.
If you give a ballpark figure or a ballpark estimate, you give a number which you think is fairly close to the actual one.
If you have a night on the town, or go out on the town, you go out for dinner and then go to a show or a dance club or some other entertainment venue.
You can say an experience is a roller coaster, or a roller-coaster ride, if it involves many emotional highs and lows, or really good times alternating with really difficult times.
You have burned your bridges, or burned your boats, if you were in a situation and you then left it after doing something that made it impossible to go back there.
You can say "we'll cross that bridge when we come to it" if someone mentions a problem that might occur in the future, but you want them to think about what's happening now instead.
If you can see light at the end of the tunnel, you can see some sign of the end of a difficult period.
If you paint the town red, you visit bars, nightclubs and other nightspots to have a good time.
If you are the new kid on the block, you are the newest person in a workplace or in an educational institute, or any other place or organization.
If something is under lock and key, it is kept in a very secure place.
You can say a problem or an experience is water under the bridge, or water over the dam, if it happened in the past and it no longer affects the present to a degree that is worth worrying about.