You play catch with them every day, but then one day, they can't catch or even pick up the ball. They don't even know what a ball is.
That's the reality for one father when both of his sons were diagnosed with an extremely rare disease called Sanfilippo syndrome.
"For a long time, I held grudges because he's taken both my sons," said Ron Brown.
For the past 10 years, single dad Brown has dug deep into his faith for answers.
"I was just shutting everybody out of the world. I wanted to be left alone and in my own little world," he said.
His sons, Christian, 11, and Isaiah, 10, are now severely disabled. They need feeding tubes to eat, wheelchairs to get around and other equipment to keep them healthy.
But that wasn't always the case for the two boys. There was a time when things were different, before they slowly slipped away.
Brown's boys used to be able to run around the playground. Now they can't even walk.
Doctors diagnosed them both with Sanfilippo syndrome, an extremely rare genetic disorder affecting only three in every 100,000 kids.
"Kids seem very normal for the first few years of their life, then develop changes in their toddler years, maybe a mental delay, a problem with seizures that's unexpected," said Dr. Natasha Burgert of Pediatric Associates.
The cells of kids with Sanfilippo syndrome don't work right, which causes the rest of the body to malfunction. Life expectancy is 12 to 20 years.
Brown decided to turn his grief into action and began advocating for the disabled across Missouri.
"He (God) wants me to fight for this and put my effort into it for my sons," Brown said.
He now carries his boys' fragile bodies to and from doctor's visits, "because the longer I can keep them healthy, the longer I keep them with me," he said.
It's a struggle for Brown to load his boys in and out of their truck. The family is in a competition to get a wheelchair-accessible van that will make life much easier for them.