Guy King wrote:

The message
from Phil Armstrong contains these words:

Just don't get sharp sand (for making concrete etc etc); you need soft
sand (or whatever builders call it).

Soft sand stains really badly - it's got a lot of fines in it - almost
clay. This is what makes it so stable when firmly compacted.

I've yet to find a good substitue for "real" play sand - apart from
beach sand.

You need to choose between two types of sand.

There is the dry sort which runs freely and works with some sort of sand
toys such as sand wheels.

The other sort of sand is often called wet sand, it's what works best
for making sand castles etc ,as it stays put when moulded.

Not all types of sand seem to work equally well as wet or dry sand. If
you can find a grade that can be used equally well either way, you'll
have done well.