Anecdotally, it is obvious that the mental effort required to perform certain activities is itself enjoyable and rewarding. For example, the very act of doing a puzzle or speculating over the guilt of characters in a crime story can feel good. However, finding evidence for this phenomenon in lab-based studies has been surprisingly difficult. We will show that in a series of studies we see that a) it is hard to know what kinds of mental effort that people will exert, b) participants in online studies are focused on minimizing mental effort, and c) even if you can get people to exert unnecessary mental effort, there are countless extrinsic reasons for them to have done so (e.g., because they might be doing the task for a reward, to satisfy a researcher, or because they are good at the task). We will also report results from studies where we see people put in excessive mental effort, but where rewards, demand characteristics, or performance cannot be the reason, suggesting that there may be a way to study the more interesting question of why mental effort can be inherently rewarding.