Animated mechanical movements
When making toys, I refer to 507 Mechanical Movements. This old book is sort of a periodic table of known mechanical movements, first published in 1868. The book has been scanned onto the web, with many of the gears animated into looping gifs so you can see exactly how their ingenious mechanisms work and what movements they create. Just paging through this amazing 507 Movements website fills me with ideas. — KK