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.