Meaning/Usage: This is used when you have to go to the bathroom. Usually referred to pee (#1) and not poop (#2).
Explanation: Having to go pee is natural. So it became popular to say "nature calls" instead of saying, "I need to go pee."
"Can you pull the car over at a rest stop as soon as you can? Nature calls!"
"I have to go to the restroom. Nature calls."
"That little boy is crossing his legs, nature must be calling."
A. "I should not have drunk all that water before we got into the car."
A. "We just left and nature is already calling."
"I need to go to the restroom."