on the tip of my tongue and off the top of my head
Are you familiar with these resources? You might find them very useful
Both of your phrases are treated on this page, e.g.:
A search for "american idioms" or "english idioms" yields several other resources as well.
Thanks so much, khardy. It helps a lot!
"On the tip of my tongue" - refers to something you know that you know, but you just can't quite remember it. It's there in your memory but you can't quite retrieve it!
For example - "I met him last year, and his name is on the tip of my tongue!"
"Off the top of my head" - this is referring to something that you're commenting on, but you don't really know if your response is accurate or not.
For example - "I don't off the top of my , but I could go and it up."
They are similar but are definitely different in meaning so no, they shouldn't really be interchanged.
Essentially, one you know that answer to but can't retrieve it there and then...the other can often just be perceived as a 'wild-assed' guess! :)
I hope that the examples help illustrate this :)
Lily, a clarification is an order on the second part of your inquirty.
Something that is "off the top of my head" simply means right at the surface of your brain, specifically information you are not digging too "deeply" to get to. It means that the information is given without preparation. It can be either because you can't/don't know the answer (this is Mike's example) OR because you know the subject well.
For example, I know I have over 50,000 known Spanish words in LingQ, but I do not know the exact number "off the top of my head." I have to check my stats.
Additionally, it can be for a subject you know well. For example, I can quickly name all 46 US presidents and their vice presidents "off the top of my head." Thanks to LingQ, I can also comfortably speak about a wide variety of subjects in Spanish with educated natives "off the top of my head" without preparation or strain on either party.