The first time Muhammad Luqman Abdul Rahman saved a life, he was on his way home from school.
Then, the Singapore Civil Defence Force's MyResponder app - which was officially launched in 2015 - alerted him that someone nearby needed help.
A factory worker at an industrial estate in Bedok had suffered a heart attack and collapsed. Luqman performed cardiopulmonary resuscitation (CPR) on the man until the ambulance arrived.
Luqman, who is now 17 and a Temasek Junior College second- year student, remembers feeling panicky at the scene.
"It didn't occur to me that doing CPR on someone would be very, very different from practising it," he recalled. "It actually felt a little daunting."
I feel that a life is at stake, and since I am in a position to assist, I should render my assistance first... Every second matters when we are in need... And of course, if we are in that situation ourselves, we would want others who are around to help us.
MUHAMMAD LUQMAN ABDUL RAHMAN, 17, a Temasek Junior College student, who has saved nearly 20 lives through the MyResponder app.
The man survived, and Luqman has since gone on to save nearly 20 lives through the MyResponder app.
Whenever someone suffers a heart attack and calls for an ambulance, app users within 400m are simultaneously alerted.
For Luqman, these emergency calls have come when he was asleep at home or even on the way to school. It was that first incident at the factory, however, that has left the deepest impression.
"He wasn't a local - he was a foreign worker who was working very hard to make ends meet back home," he said.
"It was also very relatable because he wasn't that old - he was in his 20s at the time."
At first, Luqman's parents were opposed to him responding to emergencies in this way. They were afraid that he would be blamed should something go wrong.
But he won them over by asking them to come with him the next time the alert sounded.
"I took them to one of my cases... and that was when they understood," he said.
"I feel that a life is at stake, and since I am in a position to assist, I should render my assistance first."
It is about more than saving lives though. Once, he arrived at the scene of an incident to find someone else already performing CPR.
"I saw that there were many people crowding around the patient... and I thought maybe I could help by getting people to move out of the house, so that the paramedics can enter," said the teenager.
He responds to these cases because he knows he would want others to do the same for him.
"Every second matters when we are in need," he said. "And of course, if we are in that situation ourselves, we would want others who are around to help us."
Correction note: This story has been edited for clarity.