If you're full, quite to the brim, then click the title and leave what's prim.

In the night I'm crystal clear.

In the day I disappear.

Starting small and growing down,

Then falling till I drown.

'Leave what's prim?'
'Yes, it would be proper to leave what is affectedly dainty or refined.'
'Well that's wonderful, but you didn't ask for what's proper, you asked for prim.'
'Prim is proper.'
'No, proper is affectedly dainty or refined.'
'Well to be affectedly dainty or refined is to be prim.'
'So I shouldn't be proper?'
'...You tire me.'

Riddle Answer An Icicle