Yellow thin liquid runs out when I put my head down .How come your left nostril keeps on running?

The yellow thin liquid you're experiencing when you put your head down is likely mucus, which can be a result of a sinus infection, Allergies, or a cold. If only one nostril is running, it could indicate that the affected side has more inflammation or congestion. Positioning your head can cause mucus to drain more readily from that side, leading to the sensation of a runny nostril. If this persists, consider consulting a healthcare professional for further evaluation.