Molecules float in a test tube. Their abundance (say, the concentration of some protein) becomes the input signal. And the output isn't just a cloudy solution but a clear yes-or-no answer.
The authors build a chemical cascade that does exactly that. It takes one substance with a concentration ranging from zero to one and produces a sequence of pulses, like Morse code, but chemical. Each pulse means 'on,' its absence means 'off.' In this way, molecules themselves convert an analog signal (a smooth change in concentration) into a digital code.
Errors are inevitable here, like static on an old radio. But the authors dissect where they come from and show that accuracy can be made arbitrarily high. All it takes is tuning the rate constants of the reactions: that is, how quickly one substance turns into another. In the long run, such a 'wet' computer could be embedded in a living cell, where it would monitor the cell's state without any wires.
Chemical reactions can solve differential equations — the very ones that describe a pendulum's swing or population growth. Only here, instead of numbers, you have concentrations, and instead of addition, you have mixing.
In Robert Ibatullin's novel 'The Rose and the Worm,' molecular computers grow right in a nutrient broth — almost as if predicting this work.