2 added 8 characters in body
source | link

When you closed the lid you left say 25 C air alone with boiling water. After a while they will reach an state of thermal equilibrium, where some energy transfer to the air from the boiling water, therefore as a result of increase in temperature, this air will get expanded and needs more space but there is no place to expand in the closed flask so the pressure of air will increase and when you open it, it can get free.

When you closed the lid you left say 25 C air alone with boiling water. After a while they will reach an state of equilibrium, where some energy transfer to the air from the boiling water, therefore as a result of increase in temperature, this air will get expanded and needs more space but there is no place to expand in the closed flask so the pressure of air will increase and when you open it, it can get free.

When you closed the lid you left say 25 C air alone with boiling water. After a while they will reach an state of thermal equilibrium, where some energy transfer to the air from the boiling water, therefore as a result of increase in temperature, this air will get expanded and needs more space but there is no place to expand in the closed flask so the pressure of air will increase and when you open it, it can get free.

1
source | link

When you closed the lid you left say 25 C air alone with boiling water. After a while they will reach an state of equilibrium, where some energy transfer to the air from the boiling water, therefore as a result of increase in temperature, this air will get expanded and needs more space but there is no place to expand in the closed flask so the pressure of air will increase and when you open it, it can get free.