She knows what he knows, that this heartbeat can never be taken for granted because it was made possible by a special kind of love.
"I never thought I could be so much in love with another person," Connor said.
At 17, Connor Rabinowitz had been a star catcher who was being recruited by division one schools. His life changed in 2005 when he was told he needed a new heart. Doctors said without one, he would die.
"You pray for someone to save you, but at the same time you need to pray for them to die," he said.
Kellen Roberts was a Seattle kid. He was a free spirt, whimsical and generous to a fault. He drove a red truck all over the country, living on his own terms.
"He traveled all over the world. He traveled the United States. He jammed in as much as he could," said Nancy, his mother. "It was a very different path than I would have taken, and you know what, I'm so glad he took that path. It left a much bigger legacy for us."
Connor Rabinowitz was dying. Doctors installed a three-pound metal pump in his chest to do what his heart could not. He was desperate for a donor.
"If you're willing to give up a part of your body to save someone's life, then I think it's a miracle," Connor said.
On March 7, 2005, on a trip to Souix Falls, South Dakota, Kellen hit his head in a freak accident. There was hemorrhaging and circulation was cut off from his brain. He died there, far from home, but not far from Minneapolis. He was an organ donor.
"(I felt) scared. I was going to drive to the hospital, and they were going to cut me open and take out my heart," Connor said.
The transplant was a success. The engine of life had passed from one man to another.
"I just wish he could know how grateful I am for him," Connor said.
His gratitude was such that he wrote a letter to Kellen's mother. She still has it.
"The first thing I always do when people ask me about the donor is immediately start crying," Nancy said.
Connor wanted -- needed -- to know about Kellen.
"I saw Kellen's picture, and it was, you know, very mixed emotions," Connor said. "I was so grateful, but I'd give it back in a second."
Erin has helped Connor come to terms with the fact that he is alive and that another is not. She sews hear-shaped pillows, reminders for both of them of how fragile life is. She keeps them in a jar.
"Normally, everything I make I give away, but these are for us," Erin said.
Erin, more than anyone, understands what's in Connor's heart. And here's why: She, too, knew Kellen Roberts. She was Kellen's sister.
The heartbeat she so loves to hear once belonged to her own flesh and blood.
"It's priceless to know that a part of something you loved so much can continue on, but not just continue on in existence, but be the life force for someone else, be the thing that is keeping him alive. I don't know if there's any words to describe that," Erin said.
A year after the transplant, Connor came to Seattle to visit Kellen's family. He and Erin say they fell in love almost immediately.
"Instantly, before I even walked up to him, because he hugged my mother first, and we were kind of staring at each other, over my mom's shoulder," Erin said.
The meeting is burned in Connor's mind, too.
"I have it vividly in my head. Just looking over, feeling her mom hug me, and just staring into her eyes," he said.
And so it came to be that her brother's heart is beating in her true love's chest.
To some it's a strange love, but they had to give it a shot. When you're young and in love, you follow your heart.
The impact of the heart transplant on Connor was so profound that he went to college and studied the heart. He is qualified to become a a cardiac ultrasound technician and is looking for a job in Seattle. He hasn't had any luck so far, but said he'll keep trying because it's his passion in life.