Time Travel Invention

If you could time travel and take one invention what would it be? Well, I would take a bicycle to ancient rome or greece. I mean they had roads, but most people were poor. They could not afford a horse. So most rich had chariots, but walking was the lot of commoners. The bicycle at first would be rare. People would ridicule early adopters and  call them crazy. However,  it would mobilize the poor.