You think you knew but forgot? Well press the title and write everything you remember.

A coffin, in me hold the dead,

But hide each bottom, show each head.

I sit inside, you take a look,

But outside like a tool to cook.

A vessel, as the waters go,

Not out but inside flow.

'How will writing everything I remember help remember what I forgot?'
'Well, anything you don't write, is something you forgot. So anything you don't write must be the answer.'
'Ingenious! ...I remember a marmalade sandwich.'
'Wait, comment below. I'll check it later. Only I imagine you remember a lot of useless things that I've no time to hear now.'
'Right you are.'

Riddle Answer A vase

Riddle Explanation
but outside like a tool to cook - like a pot.