CHRIS J RATCLIFFE/AFP/GETTY IMAGES

Every August, the London Zoo tracks down all 20,000 of its wild residents and holds a big weigh-in. Zookeepers use scales and rulers to take animals’ weights and heights.

Measuring the critters isn’t easy. Zookeepers use food to reward them for cooperating. For instance, staff dangled bags of meat to get the lions to stretch to their full height.

It’s important to get these numbers right, says the zoo’s manager, Mark Habben. “It helps us to ensure that every animal is healthy, eating well, and growing.”