(your) jaw drops

to show great surprise.

My jaw dropped when I heard she'd been admitted to Harvard.

• be a fully paid-up member of
• be a fully paid-up member of something
• be a happy camper
• be a hard act to follow
• be ahead of the game
• be ahead of the pack
• be a ... in the making
• be a laugh a minute
• be a law unto
• be a law unto yourself
• be a licence to print money
• be alive and kicking
• be alive and well
• be alive with
• be alive with something
• be-all and end-all
• be all beer and skittles
• be all brawn and no brains
• be all dressed up and nowhere to go
• be all dressed up with nowhere to go
• be all ears
• be all eyes
• be all fingers and thumbs
• be all fun and games
• be all heart