1] 2] 3-top] 3-bottom]
1] Helium (He) filled balloon is immersed in liquid nitrogen. When the He gas cools it a becomes more dense (it's volume decreases) and the balloon shrinks.
2] The cold-dense He balloon falls in the less dense air until it hits the floor.
3-bottom] As the He warms it expands, becomes less dense and the balloon begins to float up from the floor
3-top] The He continues to warm and expand the balloon continues to rise.