Answer:
A breath
Step-by-step explanation:
When you are in your natural environment, you naturally breathe in the air around you. But if you were to take a breath of air and hold it in your lungs, eventually you would have to exhale and the breath would leave you. Similarly, if you were to be taken out of your natural environment (such as being underwater or in space), you would not be able to breathe and would eventually die without a source of oxygen.