Why a glass is stuck to mouth when air is sucked out from glass?

When you remove the air from inside the glass you are creating a difference in pressure compared to the air on the outside of the glass. This means that the inside of the glass "wants" something to fill it up, as your mouth is there it will pull your lips into the glass. The same effect can be seen if you have a half empty glass and try to pull it put of a bowl of water. You will see that the glass will pull some of the water out with it.

The scientific principle behind this can be explain by Boyle's Law which states that "at a constant temperature, the volume of a given mass of gas varies inversely with pressure"

So if you reduce the pressure then you will reduce the amount of space the gas needs therefore something will need to fill that space, i.e. your mouth