A team led by Johns Hopkins University engineers figured out how and why human cells move much faster through thick mucus than thinner varieties. People sick with certain diseases, including asthma and COVID-19, secrete mucus that is 2,000 times thicker than normal. Cells have fin-like "ruffles" that help them sense viscosity and know when to change shape to power through the thickest mucus, they found. The findings were published today in Nature Physics.