If Simon Yam Tat-wah is speaking with razor-sharp wit these days, there may well be a literal explanation for it. Yam, who plays a professional petty thief in his new film - the aptly named, Johnny To Kei-Fung-directed Petty Thief - has prepared for his role by learning how to hide razor blades in his mouth. The actor demonstrated his new skill to the media on Monday and said it took him only a minute to master. 'I can put three razor blades in my mouth. One on the left, one on the right and one beneath my tongue,' said Yam. Not only did he not hurt himself while practising, he said, but he can now eat and speak while hiding the razor blades in his mouth. But Yam said he received a real-life education about pick-pocketing off the movie set when he lost $4,000 worth of clothing just days ago. He bumped into a stranger after buying two shirts, and although he wasn't sure if he was actually pick-pocketed or not, he later found that the clothes in his shopping bag had disappeared. As Yam must surely know from his movie role by now, even hidden weapons, or at least stinging words, are no use if you can't catch the thief.