"Brain freeze"?

[CASSIDY-FELTGEN at worldnet.att.net]

Can anyone tell me what mechanism causes the rapid, brief headache pain 
when a cold substance is ingested quickly?  Related to this is my 
question regarding the brain not sensing pain during brain surgery, but 
a person's perception of headaches as localized pain within the brain?
Is this a paradox or are headaches not actually occuring within the 
brain?