An Australian scientist came to an unusual conclusion after conducting an interesting experiment during which all participants had a glass of orange juice. The difference between them was that one half of the test subjects were really drinking simple orange juice and the other half - juice with the addition of caffeine. Then the scientist talked to each participant of the experiment and expressed several requests strengthened with more or less weighty arguments. And the result is: it turned out that two cups of coffee per 60 kg weight make "the victim" much more amenable and hence more accepting to requests in a conversation... but only at the presence of weighty enough arguments!
Of course, caffeine doesn't take away one's will completely. It just helps one concentrate and improves one's perception. So, a cup of black coffee doesn't work miracles but helps a pleasant conversation and therefore a more advantageous result of it. But don't be too enthusiastic: caffeine in large doses has an absolutely opposite effect. Upon drinking a lot of coffee one becomes absent-minded and is distracted by every small thing.
So, you have an important negotiation, you are going to make a deal or maybe even offer your hand and heart? First, offer a cup of coffee! And you will have all the chances for success.