Let's Not Sleepwalk Into New European Lockdowns

This brainteaser is how Martin Hirsch, head of the Paris region’s hospital network, describes the brutal first wave of Covid-19 that triggered lockdowns across Europe in March and April. The answer is 28, because once the pond is half-covered it only takes one extra day for the lily pads’ spread to double in size. “One day’s delay means double the cases, double the seriously ill, and double the deaths,” he writes in a new book about hospitals battling the virus. “Every day counts.”