anti-thumb sucker

so at the check up with the doctor a week or so ago the doc said that Rigby had to stop taking a passy but also that Adler needed to stop sucking on her thumb. i don't know HOW to break that habit. We have done well with getting rid of the passy for Rigby she does not even ask for it anymore. But how do you tell a 5 month old to stop sucking on her thumb??? UGH.