You certainly have heard the expression, "There's no such thing as a free lunch." Though the Oxford Dictionary of Proverbs points out that this was originally a colloquial watchword in economics, the expression has moved into broader use. And as the expression has evolved in everyday parlance, it has wandered from its meaning as economists use it.
In the contemporary vernacular, when people say that there's no such thing as a free lunch, they probably mean something like "if someone offers to give you a free lunch, you'd better watch out because there is probably a catch, and the person offering the lunch will want something from you after all."
But that usage misses the point. Suppose that someone offers an economist a free lunch. When the economist laments that no lunch is really free, what he means is that opting to eat the free lunch means not doing something else instead. For example, by eating the lunch the economist consumes his own time (with which he could have done something else), he consumes calories (perhaps he is watching his waistline), and he consumes space in his stomach (that could have been filled with alternative eats). Not to mention that he will also "consume" the company of the person offering the free meal--company that might be pleasant or unpleasant.
So the "no free lunch consideration" really is this: If you accept a free lunch offer, how costly will it be to you personally to accept, given what you would be doing instead if you did not accept? Because you always give up something to accept an opportunity like a free lunch. The only question is how valuable the free lunch looks in comparison to your next best option.
Having said all that, here's a tip: there's free ice cream at participating Häagen-Dazs® shops today from 4-8 p.m. And only you can decide whether it's worth the time and calories to take Häagen-Dazs® up on its offer.
(Hat-tip: Ruth Arevalo)