Rocks in Bucket
Start with a bucket, some big rocks enough to fill it, some small stones, some sand and water.
Put the big rocks in the bucket – is it full?
Put the small stones in around the big rocks – is it full?
Put the sand in and give it a shake – is it full?
Put the water in. Now it’s full.
The point is: unless you put the big rocks in first, you won’t get them in at all.