Wednesday morning I went to the funeral of my husband’s grandfather, who had lived 93 years. As a couple of dozen family members circled around his grave site, I couldn’t help but think of how bizarre and disorienting death is. Just a few days earlier, there was, there existed, a physically robust, smiling, warm, breathing man. And now his big body was somehow fixed in a wooden box, descending into a dirt hole just a few feet from his tearful widow, children, grandchildren, and great-grandchild.

My niece Emily, who’s almost 3, was on her mom’s hip, snacking on Cheerios and watching the burial intently. “What are we doing?” she said. “Saying good-bye to Opa,” her mom whispered. “Bye-bye, Opa!” Emily said cheerily. Her mom burst into tears. “What’s wrong, Mommy?”

It was one of the morning’s many bittersweet moments, a reminder that even amidst death, life goes on. I kept thinking about it throughout the day, as I saw Emily laughing and climbing and running around an apartment full of grievers. When does a child learn the concept of death? And how do scientists even figure that out?

Turns out that psychologists have been investigating children’s ideas of death since the 1930s. When judged through a modern lens, some of these early studies seem a bit wacky. In the first, published in 1934, doctors interviewed boys living in Bellevue Psychiatric Hospital in New York. As part of the interview, they recorded the boys’ responses after a doll fell to the ground with a loud noise.

One of the most famous early studies was done by Hungarian psychologist Maria Nagy. She interviewed nearly 400 children living in Budapest just after World War II, a time when death was everywhere. She simply asked them to answer, either in words or pictures, “What is death?”

Just one study looked at this topic in the 1950s, followed by eight in the 60s and two dozen in the 70s. Almost all of these studies, according to a fascinating review published in 1984, relied on interviews with children. Some, like Nagy’s, asked open-ended questions, whereas others were more specific, asking things like, Can a dead person come back to life? Can you think of someone who might not die? Will you die?

No matter what your age, death is not easily defined. But for the purposes of research, scientists define a child’s understanding of death by looking at three specific aspects of the concept.

The first is death’s irreversibility. Once your body is dead, it cannot ever be alive again. Kids under 3 don’t understand this idea; they’ll talk about dead people as if they went on a trip or took a nap, or will hold open the possibility that dead things can come back to life with the help of water, food, medicine, or magic. Children begin to grasp death’s finality around age 4. In one typical study, researchers found that 10 percent of 3-year-olds understand irreversibility, compared with 58 percent of 4-year-olds.

The other two aspects of death are learned a bit later, usually between age 5 and 7. One, dubbed ‘nonfunctionality’, is the idea that a dead body can no longer do things that a living body can do. Before this is grasped, kids will affirmatively answer questions like, Can a dead person feel? or If someone died, could he still eat? Can he move? Can he dream?

Then there’s death’s most befuddling attribute, at least for me: its universality. Every living thing dies, every plant, every animal, every person. Each one of us will someday expire. Interestingly, before children learn this, many believe that there are certain groups of people who are protected from death, like teachers, parents, and themselves. “Without a doubt, most children understand that some people die before they understand that they themselves will die,” the review authors write. And even children who understand that they will one day perish “have a tendency to say that their death will occur only in the remote future when they get old.”

These are all generalities and tendencies. Some kids develop more quickly than others. And some studies have found that emotionally traumatic events — such as the loss of a parent — can speed up a child’s understanding of death.