system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why I couldn't stand at a post office all day making money off the conversion rates?

Right now the £ to $ is 1.57 and the $ to £ is 0.64.

So if I have £500 and change to dollars I have $785 then if I change that back at 0.64 I would have £502.4? Why could I not just stand there making 2 pound every minute or so?
gpt
It's not free to convert currency is the answer.  

Also, there are people that do this all day.  They're call currency traders.  There is whole exchanges, software packages and trading desks setup just to trade funds.