Thunk. The mini marshmallow—Molly’s favorite treat—landed next to her. When my 10-pound shih-tzu heard it land, her head whipped around in the direction of the sound and she inhaled it, not chewing it even once.

“You missed,” I said. “Try again.” I tossed another marshmallow. This time, it bounced off her nose and fell to the floor. She chased after it and ate it.

Some days you don’t catch all the marshmallows. But it wasn’t just that: Over the last three years of her life, Molly stopped terrorizing rabbits in the backyard. She stopped sleeping on the stair landing. I didn’t realize that she had started going blind, that her once-expressive eyes would turn milky white as she lost most of her sight.

For anyone who has ever loved her dog, watching it go blind can be heartbreaking. Some lose their sight entirely. Some, like Molly, lose part or most of it. They may be able to see shapes, shadows, and light or they may be nearsighted. In most cases, the dog will need its caretakers to take precautions and make special allowances—to be its seeing-eye human.

Kristen Murdock, a foster dog mom with Blind Dog Rescue Alliance, says the best way to help a blind dog is to get to know its personality and limitations. Some dogs can see shadows and light, but their depth perception is off. Laying down textured area rugs near steps can give a dog a warning that something is changing and they need to be aware of it.

In the city, it’s important to keep closer tabs on your vision-impaired dog while on walks. Murdock says she keeps the dog close at first, then gives it a little more lead as it adjusts. The pups that she fosters wear bandanas that say “I’m blind” so others can be aware, too. If a person or dog approaches, she’ll speak up about her dog’s blindness, sometimes asking the person not to approach. Murdock also never uses a retractable leash or lets her dog off its leash, even at a dog park. The dog can become too overwhelmed.

But dogs that lose their vision can usually adjust at home, says Steve Smith of the Rolling Dog Farm, which cares for special needs animals including blind dogs. Most dogs build a “mental map” of their surroundings, and even blind dogs that lose their sight overnight can remember where things are, after a brief period of reintroduction. Blind dogs remember where their beds and food and water dishes are, as well as the front door. After Molly lost her sight, my mom held off on rearranging furniture so she could still have full rein over the house.

The most common cause of blindness in dogs is cataracts, says Chicago vet Dr. Barbara Royal, owner of the Royal Treatment Veterinary Clinic. Molly got hers from old age, though they also develop due to poor diet. Detached retinas—usually caused by high blood pressure or kidney disease—can also cause blindness, but retinas can actually reattach on their own. With a good diet and plenty of exercise, Royal says the problem may clear up by itself.

A dog’s breed can play an important role, too. Sometimes blindness is preventable, like when a careless breeder doesn’t do enough research and ends up with a litter of blind puppies, Murdock says. That’s what happens when you breed two spotted-coat Australian Shepherds together, for instance. On the other hand, SARDS (sudden acquired retinal degeneration syndrome) causes sudden blindness and vets don’t know how to stop it. According to Pet MD, it’s more likely to affect dachshunds, Brittany spaniels, and miniature schnauzers. It can strike purebred and mixed-breed dogs, usually between the ages of 6 and 14. Royal says her practice commonly sees blindness in poodles as well.

As Molly started to lose her sight, she missed more marshmallows than usual. Though Molly never liked being picked up, my older sister Jill found it easier to sneak up on her and scoop her up. In the mornings, Molly usually ran at top speed down the stairs and jumped the last four. When she lost her sight, Molly sat and waited for Mom to carry her down. We all kept an ear out for her whining in case she got stranded up there.

Both Royal and Murdock say the most common sign of blindness is the loss of the ability to go up and down stairs. Early on, some dogs bump into furniture or seem confused and unable to find important things. Others seem to lose their sense of direction or they may become more reactive to other dogs and people. Some even change their sleeping patterns, pacing all night and sleeping all day because they can’t tell night from day. Smith says there may be a discharge coming from the eye, or the dog’s eyes colors may change.

Molly didn’t seem to care that she couldn’t see. When Mom grilled hamburgers, Molly didn’t need to see the patties; she smelled them as soon as they hit the grill. When I walked her, she still insisted on that same stupid walking route. If anyone needed the adjustment, it was my parents, my older sister, and me.

In her later years, Molly had more accidents than Mom preferred. We stopped waiting for her to ask to go out and started taking her out every few hours. The last time I walked Molly was a few months before she died. Layered in two sweaters and still shivering in the August sun, Molly now had cancer in addition to her partial blindness. When I set down my bag and called her, her tail wagged, and she licked my face.

She still pulled at the leash on our walk, but she nearly crashed into a mailbox. Later, another dog ran up to sniff her, and she jumped two feet back.

But as we sat in the grass looking over the lake, the setting sun shining through her vacant eyes, she looked like a puppy again.