When you drink a small amount of something, you sip it. If your cocoa is extremely hot, you should probably sip it until it cools off a little bit.
You may sit in a corner and sip your punch at a dull party, but when your long hike leaves you parched, it's hard to sip from your water bottle instead of guzzling. As a noun sip means "small drink," like a sip of Coke or a sip of soup. The origin of sip isn't clear, though we know it's related to the Old English supan, "take into the mouth a little at a time."