Splint re-ignites with a pop sound

Reason:

When a glowing splint is lowered into a gas jar of oxygen, the splint re-ignites with a pop sound. This is because oxygen is a supporter of combustion, and when the splint comes into contact with the oxygen, it provides the oxygen needed for the splint to continue burning. The pop sound is caused by the sudden release of energy as the splint reignites.