At one point, Bestovius gives the item to Watchitt, but he never returns it, despite him asking for it back time and time again. When Mario arrives at his house to collect the Training Machine for Merluvlee, Bestovius asks him to get it back; however, because Bestovius refuses to reveal any details about it to Mario, he simply tells him to ask Watchitt to give him the "You-Know-What," hence the name given to it as an item. The You-Know-What can only be obtained from Watchitt after obtaining Merlumia's autograph for him. Once Bestovius is given the item, he gives Mario the Training Machine.
Only after giving it back to Bestovius does he reveal a single detail about the You-Know-What: it is a collector's item.
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