Social engineering challenge: Stop!

My challenge for the past week was to have a fellow player fail their challenge due to my efforts. I had no definite plan on how to go about doing this, so i just had to keep my eyes open for when i saw someone openly trying to accomplish their task. the only open attempt was made by Raven Blue when she stuck the piece of tape to MzQueens water bottle. It all happened do fast i couldn't process anything she managed to stick tape on the bottle before I could even get up to intercept, so that was a fail. Before that, when i saw CaptianBlack take his challenge card from the bag, there were only two card in there. When i picked mine, it was the same challenge i had in a previous week, so i put it back. That left two cards in the bag, one of which i knew what it was so that was a 50-50 chance he'd pick that card. when he opened his card, i noticed the color of the writting and it was the same as the one i had, so i knew what his challenge was before he even finished reading it. for the whole class period, i tried to keep my eye on CaptianBlack to try and watch out for his attempt at his challenge so i can stop him. unfortunately, i didnt see him attempt his challenge, so that was a fail. looking back, i probably should have just said out loud to everyone what his challenge was, that way everyone would be suspicious of him trying to trade anything with them.