A casual English expression you can say when you are asked a

question suddenly.

“On the spot”

Meaning, suddenly.
No time to think.

You are in class.
Your teacher suddenly tells you to stand up and asks you for your ideas.
You can’t think of any ideas.
You can say:

“I can’t think of any on the spot.”

Meaning, I can’t think of any right now.
But I might have some ideas after some time to think.

“On the spot” means “right now”.
Usually used when responding to a question, and you don’t have any ideas.

“On the spot” can also mean “suddenly”.
Suddenly asked something in front of a group of people.

“The boss put me on the spot”
Meaning, suddenly asked me a question in front of everybody.

“The teacher put me on the spot”
Meaning, suddenly the teacher asked me a question in front of the whole class.

Have you ever been asked a question suddenly in front of a group of people?
Have you ever been “put on the spot”?

Thanks for reading.