Have you ever stopped to think about how we are able to suck liquids through a straw. What exactly makes the liquid climb up the straw and into our mouths?
When you drink from an open glass of water, air pressure allows the water to travel up the straw. By sucking on the straw you are reducing the air pressure inside your mouth. While sucking on the straw, the air pressure in your mouth is less than the air pressure outside of the straw (in the room, in the glass, etc.). The outside air pressure is pushing down on the water which forces the water up the straw. But when air pressure on the water is blocked (when you seal the jar lid), there is no air pressure to help push the water up your straw. The air can't get to the water to push on it, so it doesn't go up the straw. Now matter how hard you suck, the water will not go up the straw.