Why does the currency drop when the stock market rises?

I’ve often noticed that when the U.S stock market is on the up the USD tends to be trending down and the same for the FTSE. I mean the FTSE 100 is so strong right now compared to the GBP and I don’t really understand why this is.

The same for the Dow Jones and the USD.

In the case of the FTSE-100, it’s mostly because of the preponderance of companies within that index who export and therefore earn a significant proportion of their money in [I]other[/I] currencies: a weaker pound helps them by increasing the Sterling value of their takings. (It’s more complicated than [I]just[/I] that, of course.)

I think lexys has it right. But I don’t think it gets much more complicated does it? e.g. when the USD becomes “cheaper” for a non-US investor to buy dollars in order to put money into the US economy, then they can buy more dollars per yen or Euro or whatever currency they hold now than when it was higher - so, USD down, Dow up: USD up, Dow down. Works most of the time.