It is necessary or not to communicate with sellers, waitresses and neighbors on the plateau. People, you have fallen from the moon, what, about what at all? What you need is to understand non-verbal (and sometimes verbal) signals about whether a particular person wants to shake with you or not. Someone will pick up the question, and it will fall, and someone will answer unambiguously, if they answer at all - and you should already understand everything. Stop talking about spherical sellers in the vacuum - people are all different.