We've all been there. You've had a long day at work, and you're sitting down to enjoy a drink with one of your German friends. Before you take your first sip of sweet, sweet Helles, you raise your glass, as humans have done across the world since time immemorial. 

You clink glasses with your friend and utter a hearty “Prost!”. Then suddenly your friend's face darkens. You have committed the biggest faux-pas in German drinking etiquette: you have failed to make eye contact as you clinked your glasses together. 

Perhaps you have just arrived in Germany and were unaware of the custom. Perhaps you are British and therefore hard-wired to find extended eye contact highly uncomfortable. Perhaps you are with more than one friend, and you simply couldn't face leaning at full stretch, table corner digging into your pelvis, to clink glasses and pointedly stare into the eyes of the people on the other side. 

Whatever the reason, you have upset the Germans. For them, clinking glasses and making eye contact is an absolute must whenever two or more people are gathered in the name of alcohol. 

Ask them why, and they will give you the answer everybody knows. If you don't make eye contact, you are in for seven years of coital misery. 

Yet that is a superstition, not a reason. “Look me in the eye or you will get seven years of bad sex” is only really a credible argument if it comes from the mouth of the person you share a bed with, and that person is really committed to delivering on promises. 

In truth, as with all these things, nobody can say for sure where, when and why the custom originated, and why it persists in Germany and elsewhere in Europe. There are, however, a number of theories, most of which relate to medieval times. 

One suggestion is that the clinking of glasses mimics the fanfare which would have played when a King raised his glass to begin a feast. Yet that doesn't explain the eye contact rule. 

The most credible explanation is that clinking glasses is an insurance mechanism against being poisoned. If the person you are drinking with had poisoned your drinks, bumping your glasses together, particularly if done with some force, would mean that the drinks would splash into one another, and your potential murderer would risk killing themselves along with you. 

So why the eye contact? The only way to be sure that the poison had not spilled into his glass would be to watch the glasses as they hit each other. By making eye contact at that moment, the two drinkers assert to one another that there is no reason to look at the glasses, establishing a mutual trust that neither drink is poisoned. 

How much genuine historical evidence is there for this theory? Not a great deal. But it is at least harder to disprove than the seven year rule. And after all, taller tales have been told after a few beers. 

So there you have it. Even if you are not worried about seven years of unsatisfactory intercourse, you might be worried about being poisoned. So embrace the eye contact and give your glass a good old bump.

READ MORE: 8 strange superstitions that the Germans hold