My first machine was a 2-axis egg plotter (see right), the "eggbot." About 10 years later when I was asked to teach a two week course to children about motion control, I returned to eggbot (read more in the eggbot page under teaching machines).
Since then, my skills with building machines have vastly improved, and I have built several egg plotters which are far faster, more accurate and larger than the first egg plotter. I use these "ovagraphs" mostly for demonstrations, such as during the Maker Faire, because I think the simplicity resonates with any audience, regardless of age.
A few sample pictures of the ovagraph at work:
See more examples in the Miscellaneous Pieces Gallery.