So are you an introvert or an extrovert? Or are you - like me - unsure, and rather sceptical that such categories are useful? Well whatever, this is a fun test.
Lemon juice is literally mouthwatering; when you drop a bit of it on your tongue, your body produces saliva. Some psychologists think there's a particular system in the brain the Reticular Activating System or RAS, that responds to stimuli including food and social contact. Their theory is that introverts have more RAS activity than extroverts, even when it's not stimulated, and that a small stimulus produces a large response. Similarly, when introverts meet people they have a stronger response (which I guess includes nervousness and anxiety and a feeling of please leave me alone). Extroverts, on the other hand, produce less saliva in response to lemon juice and are more comfortable with social contact. The BBC has a handy article explaining how to carry out this test on yourself.
Maybe this is what Thom Yorke was doing when he woke up sucking a lemon.