This should always be shown to Thai royalty and monks. Never do anything stupid like lighting a cigarette with a Thai banknote. The King's head is on the note. You have just insulted the King. I once dropped my wallet, and playfully put my foot on it; I thought my Thai girlfriend was about to grab it. Without knowing it, I had caused offence. I had just placed my foot on the king's image. They will not get nasty with you. They know that you don’t know. It was very diplomatically explained to me.