At the end of a busy season, the current compost pen is often well filled, and it's very tempting to top it off with the grass from that final cut - which, this year, was in December for many of us! - then leave it over the winter to get on with composting itself.
This is all well and good as long as you don't leave the grass piled into a conical heap, as per the first picture above. All this does is create a nice waterproof thatch to the pen (after all, thatching is done with straw or reeds, which are both, in effect, long versions of grass) which prevents any water getting in, and the conical shape just helps the water to run off even more quickly.
In all the years I have been gardening, I have seen far more compost heaps that fail for being too dry, than ones that are too wet: dry material won't rot!
An additional problem is that the thick layer of grass also heats up for a brief period after you pile it on, so you get a layer of dried-out material which is even more impervious to water, and which kills all the worms which would otherwise work hard all winter to make the compost.
Whenever I find a compost heap that has been piled up like this, I take the time to rake the grass out towards the edges, stuffing it down well into the corners, and making a depression in the centre, so that any rain will run into the pen, not out of it.
You can see this in the second picture: much better!
I also push a few vertical holes in the grass layer, so that the water can run through it, instead of sitting uselessly on top of it.
This means that all the winter rain will go into the pen, and by spring it will be well on the way to being processed into good useful compost, instead of sitting there going stale all winter, with little worm activity to keep it aerated.
So pop out and check your compost heaps before it gets too cold: make sure they are more or less level on top, with a central depression, and no big gaps in the corners.
Then you can go indoors and enjoy being inside through the worst end of the year (weatherwise), knowing that your garden waste will be gently turning into compost, while you are staying snug, and looking at seed catalogues!