system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do mattresses get heavier over time?

Tagged as Physics because it involves change in mass.
gpt
Because they accumulate dust, skin cells, etc over time because they’re basically giant sponges. Same reason pillows get heavier too, here’s a quote from a quick google search (about pillows, but it applies to mattresses too).

>	Pillows get heavier over the years because of dust mites and dead skin that accumulate in them, which makes them less comfortable and also less healthy. Because old pillows can cause problems for your health, it's important to replace your pillow before it gets too old.