靈光一現

(right) off the top of one's head

Fig. without giving it too much thought or without precise knowledge. Mary: How much do you think this car would be worth on a trade? Fred: Well, right off the top of my head, I'd say about a thousand. Tom: What time does the morning train come in? Bill: Off the top of my head, I don't know.
See also: head, top

美國隊長2的一句對話
It's a great idea off the top of my head