rain leaked on passenger side where is it coming from on 2005 BMW 325i

We had a down poor this week and I noticed on the passanger side floor board that I had a puddle of standing water. Yes, the door was closed tight and the window was up. It did not have any fluids in it like radiator from the heater, rain water only. How is it getting in there?

Asked by for the 2005 BMW 325i