Guessing Activity - WHAT AM I?

I have no colour,
not even white,
but sometimes I'm wide and blue.
Sometimes I'm still,
and sometimes I rush,
and I can fall down on you.

I can change my shape,
I often do,
but I'm hard to hold in your hand.
You need me to make
your castle stand up
when you're playing in the sand.

I haven't much taste
but on a hot day
your tongue's hanging out for me.
Then I am cold,
but I can be hot
When I'm in your mum's cup of tea.

What am I?

Jo Peters in The Works, Chosen by PAUL COOKSON