Why flame goes upwards not sideways or down when we light up a matchstick?

Flames rise because of convection currents. When a fuel is ignited, it produces hot gases that are less dense than the surrounding air. This causes them to rise, creating a flow of hot air that carries the flame upwards.

The shape of the flame is determined by the rate at which the fuel is released and the amount of oxygen available. A tall, narrow flame indicates a slow release of fuel and a limited amount of oxygen, while a short, wide flame indicates a rapid release of fuel and an ample supply of oxygen.

The upward movement of the flame is also affected by the Earth's gravity. The force of gravity pulls the hot gases down, but the upward force of the convection currents is greater, so the flame continues to rise.

In addition to convection currents and gravity, the shape and movement of a flame are also influenced by the surrounding environment. For example, a flame in a windy environment will be blown sideways or down, while a flame in a still environment will rise straight up.