1. Pressure = Force/Area
When a ballon is pressed against many pins then the area is increased as compared to when it is pressed against a single pin. The force by which the balloon is pressed remains the same. Less the area, more the pressure. So, the balloon pops when it is pressed against one pin.
2. The candle goes off when covered with a glass because for burning the candle need oxygen. When glass on placed on the candle then the oxygen supply is stopped. So, when available oxygen is finished, the candle goes off. Water rises in the glass, because when the candle is burning, it expands the air inside the glass due to its heat. But when candle wents off, the air is cooled again and it contracts. So, a vacuum is created. That's why the water rises in the glass to fill the vacuum and to balance the air pressure inside and outside the glass.
3. Objects tend to maintain their state of motion or rest unless subjected to an external force. This property is called Inertia. More the mass more the inertia. When sudden brakes are applied, then our body still tends to remain in motion. That's why our body go forward.
