nature of the beast

answer the call of nature

To satisfy a bodily urge to urinate or defecate.

beast with two backs

Two people united in sexual intercourse in the missionary position.

better nature

the more amiable qualities of someone

let nature take its course

To permit events to proceed or a situation to develop without intervention or interference.

nature abhors a vacuum

Generating and maintaining a near-vacuum is challenging (requiring technical rigor) because ambient pressure tends to push matter forcefully into the evacuated chamber; and generating a true vacuum (devoid of any matter at all) is extremely challenging.

nature calls

Used to indicate that someone (usually the speaker) feels a need to urinate or defecate.

second nature

A mindset, skill, or type of behavior so ingrained through habit or practice that it seems natural, automatic, or without a basis in conscious thought.