How do we know we are alive?

When you ask Google this question, this is the answer you are presented with. 

Some might say it is suffering, as the general consensus is that only living things can feel pain.

There are other ideas, more than anyone could possibly count. 

In the end, we each decide how we define our lives. Surely that means we choose whether or not we can truly be said to be alive. 

So what would you say? How do you know you are alive?