system
Given the following post, respond as if you're commenting on Reddit:
human
If liquid helium is the coldest substance we have commercially available to us today, how did we first cool helium down to that temperature without the use of something colder?
gpt
Use a refrigeration cycle. Compress it and raise it's pressure (and thus temperature), cool it (reject heat) while at a relatively constant pressure, then expand the gas back to ambient pressure, thereby cooling it further.

Repeat this process using different coolants. Eventually: compress it a lot, cool with liquid nitrogen, expand it back out to some ambient pressure and it will condense into liquid helium.