Way back when, in 1998, a student on my year abroad in Argentina, I went round to a friend’s house. She cooked, we might have smoked a cigarette, and then she offered me some green leaves, served up in a cup. Hoping it was some new kind of illegal substance I could boast about back home, it made me a little dizzy although I wasn’t hooked from the first sip. And as she patiently explained the origins of this drink to me, I suddenly realised that yerba mate wasn’t some illicit herbal secret, and that all the Argentines were at it…