Liquid nitrogen has a temperature of -196 °C. Liquid nitrogen is poured over an inflated balloon. Observe, that only a thin skin-like object is left over from the balloon. When it is warmed up, the balloon regains its original volume and shape. A bit more spectacular effect is achieved with a helium-filled balloon. At first the balloon tends fly up in the air, but it is held in position by a weight. When cooled by liquid nitrogen, the balloon shrinks (not as much as the air-filled balloon) and it sinks down on the table. When heating up, the balloon slowly rises and flies up in the air again.
Explanation 1: The volume of the balloon decreases by the low temperature, because the gas inside is cooled down. At room-temperature the helium balloon is lighter than air with the same volume of the air, which makes it go up. At low temperatures its volume is too small, so it is heavier than the same volume of air.
Explanation 2: In the balloon there is an isolated amount of air or helium. By the low temperature the pressure of the gas inside the balloon tends to decrease. The atmospheric pressure, however, forces the balloon to a smaller volume until the pressure inside and outside the balloon are equal (neglecting the surface tension of the skin of the balloon). At these temperatures air can be compressed more than helium, because it contains components than can be liquified or frozen (nitrogen, oxygen, carbon-dioxide, water). At these temperatures helium always will be a gas. When heated up, the pressure in the balloon rises, the balloon expands, and will reach to its original shape.
The helium-filled balloon flies up in the air, because the average density of the balloon and helium together is lower than the density of air. Just like a piece of wood, held under water, tends to rise to the surface of the water. When the balloon is cooled, the volume of the balloon is greater, resulting in a greater average density of the balloon and helium. This makes the balloon come down.