I know my boyfriend is about to propose to me, how do i act suprised? Any tips?

My boyfriend of almost 2 years is going to propose to me.... how do I know? Because my completely loving but insanely air headed boyfriend asked me to check a transaction in his bank records and i saw that there is a $2800 transaction to Kay Jewelers... I'm so excited but i'm worried when he asks me that i wont be able to pull off a believable surprised face.... Any tips Ladies and Gentlemen?