What does the idiom running out of steam mean?

"Running out of steam" means losing energy, enthusiasm, or momentum. It's often used to describe a person or a project that has been going strong but is now starting to slow down.

Here are some examples:

* "After working all night, I was running out of steam by morning." (Meaning: The person was tired and losing energy.)

* "The campaign was initially successful, but it's starting to run out of steam." (Meaning: The campaign is losing momentum and effectiveness.)

* "The conversation ran out of steam when the topic shifted to politics." (Meaning: The conversation lost interest and excitement.)

Essentially, "running out of steam" is like a car running out of fuel - it can't go any further without a boost.