What Are Mushroom Gills For?

I am sure The Cygnet would love to know the answer - after all he asked me when I took him out for a walk this afternoon............... and I have absolutely no idea.

He also asked what the water on the leaf was for ............ just before he squished the drops.

Although he was wearing his wellies I still had to change his trousers when we got home - his puddle jumping went slightly 'extreme' ............ look at that cheeky face