Have you ever tried this?
It is not at all easy to do.

Philip could not do it at all.

He held his breath and he tried and he tried, but he only felt fatter and fatter and more and more as though in one more moment he should burst.

So he let his breath go.
'No,' he said, looking at his hands; 'I'm not any more invisible than I was before.

Not so much I think,' he added thoughtfully, looking at what was left of the cherry pie.