Spawn a Time Machine and play around with it for a little while. Eventually, a random event will occur, in which instead of going to the era you have selected, you will go back to Level 1-1 from the original Scribblenauts. There, you will see a past Maxwell loitering around the stage and your magnifying glass will not be able to identify him. You can interact with him the same way you interact with anything else, including killing him. Also, if you play with the Time Machine a little more, you will be sent to a stage in which you will witness Maxwell exiting the stage through his own Time Machine. When he leaves, there will be nothing else for you to interact with in the stage.