system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do highly precise mass scales fluctuate wildly?

I was preparing some samples today for dissolving, and I needed to prepare 0.001g and 0.0005g of material. The scale I was using had 5 decimal places. 

Anytime I put a sample on there, it would go something like 0.00118g, then a few seconds later, 0.00090g, then another few seconds, it would even hit 0.0005g. That's over 50% in fluctuation!

What are the machinations of this phenomena? ELI5!
gpt
When you are trying to measure that precisely, even small variations in the environment can have a profound effect.  Those scales measure the downward pressure on the plate to determine weight, so even a slight breeze from the labs A/C could put enough pressure on the plate to change the reading (that's the reason they have those sliding doors)