@Bridget Ha, ha. I think I was with you the day we got checked in Berlin not only on the UBahn and the bus but I've now got one to top that. This summer I was checked twice on the same tram within 5 or 6 blocks by two different controllers.  Then I got checked again shortly there after on a different tram line.  It was a pretty exciting day--and luckily I had tickets for both rides.   I think that when you're tall and blond the controllers look at you and think that you must not understand how the tickets/validation/bus-riding works.  But I know: buy ticket, stamp it, when you change to a new line then you have to stamp a new ticket (this part is very important) and if you have a booklet of tickets you must use them in order from 1-10.