A friend showed me this, which is from here. You're supposed to focus past the screen so you see three circles, where the center one is an overlapping of the two. Now your brain needs to decide what to do with the center one — see a vertical line, a horizontal line, or both. When I do it, my brain kinda flip-flops between seeing one and then the other, and sometimes both. The guy who created this advocates trying to see both.

Some thoughts:

• It seems that I have some voluntary control over which image dominates, which is odd.. I hadn't even considered trying to control this, but it does seem possible with practice, and I wonder if that same mental knob is useful for anything else..
• This makes me think of eye dominance. I'm left eye dominant, though I don't really notice a dominance of the left vertical bar in this exercise.
• I like this exercise because it gives really good feedback about how well I'm controlling a mental knob, even if it might be a useless knob.. but it seems like good practice finding a mental knob, and I wonder what other exercises like this are possible for developing mental skills.