What do all living things inhale during respiration?

Question

What do all living things inhale during respiration?

Answer 1

Oxygen

Answer 2

The vast majority of living organisms, and almost all multicellular organisms inhale oxygen during respiration.

However, not all organisms breathe oxygen.  There are quite a few to whom oxygen is actually poisonous.  There are also organisms that use sulfur as the final electron receptor in their repiratory process - mostly bacteria, but there are some multicellular animals that live near deep sea thermal vents that do it too.

There are also a large number of organisms that undergo not only cellular respiration, but also produce energy through photosynthesis.  Photosynthesis requires the intake of carbon dioxide, and oxygen is actually their waste product.