Briefing A Group

A ranger was briefing a group of walkers in a park:
"It is possible," he warned, "that we will encounter a grizzly bear.
However, as grizzlies usually avoid contact with humans, I suggest you attach small bells to your backpacks to signal your approach & give the bears time to retreat.
If you do see any grizzly bear droppings," he added, "leave the area at once."
"But how will we know if they are bear droppings?" asked one walker.
"Easy," replied the ranger.
"Grizzly bear droppings are full of small bells."