from the knowledge you have in your memory

中文解释

凭既有知识；不假思索地 \"What\'s the capital of Mauritania?\" \"I don\'t know off the top of my head, but I could go and look it up.\" “毛里塔尼亚的首都是哪里？”“我没法马上告诉你，但我可以去查一查。”