The former happens fairly regularly, while the latter never happens! Another explanation which is certainly true of some occasions is that when we feel slightly nervous we often don’t know what to do with our hands, so we are constantly trying to do something with them. If we have a drink in them, we are likely to be constantly sipping it. So for example, in a situation where we are standing socialising with people we may not know well (possibly business contacts) and we have a drink in our hands and nowhere to put it down, we will tend to keep sipping it.
keep sipping when hands drink
