system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the water from a plastic bottle taste fine after being on the store shelf for months, yet the day after I put my own tap water in there it tastes of plastic?
gpt
Here are a few possible reasons:

bottled water may have additives in it to enhance flavor(no, really, pure water tastes funny, most water we drink has some levels of minerals dissolved in it that alter the flavor)  
your tap water's chemical composition might be better at leaching chemicals from the plastic (more corrosive or the dissolved minerals interact with the bottle material)  
If your tap water drunk from the bottle is warmer than it is from the tap, it could already taste like that, but the coldness masks the flavor when it's straight from the tap.