If you want to really seal an envelope securely and avoid licking its adhesive underside, all you need is a little nail polish.
Reader's Digest suggests that nail polish secures an envelope better because it prevents any recipient from steaming it open. It also keeps you from licking it. So if you want to make sure nobody sneakily opens your letter (or just want to save your tongue a little trouble), nail polish should do the trick.
38 Clever Uses for Nail Polish [Reader's Digest (Page Removed) via TipHero]