On February 24, 1988, Richard Lenski seeded 12 flasks with E. coli and set them up to shake overnight at 37ºC. But he seeded them with only enough nutrients to grow until early the next morning. Every single afternoon since then, he (or someone in his lab) has taken 100 microliters of each bacterial solution, put them into a new flask with fresh growth media, and put the new flask in the shaker overnight. Every 75 days—about 500 bacterial generations—some of the culture goes into the freezer.
The starvation conditions are a strong pressure for evolution. And the experiment includes its own time machine to track that evolution.
The pivotal piece of technology enabling this experiment is the -80ºC freezer. It acts essentially, Lenski says, as a time machine. The freezer holds the bacterial cultures in a state of suspended animation; when they are thawed, they are completely viable and their fitness can be compared to that of their more highly evolved descendants shaking in their flasks. As an analogy, imagine if we could challenge a hominin from 50,000 years ago to a hackathon. (Which she would probably win, because the paleo diet.)