Replace the question mark with a number

They had been up in the attic together doing some cleaning. The kids uncovered an old manual typewriter and asked, 'Hey Mom, what's this?'
'Oh, that's an old typewriter,' she answered, thinking that would satisfy their curiosity.
'Well what does it do?' they asked.
'I'll show you,' she said and returned with a blank piece of paper. She rolled the paper into the typewriter and began striking the keys, leaving black letters of print on the page.
'WOW!' they exclaimed, 'That's really cool.! But how does it work like that? Where do you plug it in?'
'There is no plug,' she answered. 'It doesn't need a plug.'
'Then where do you put the batteries?' they persisted.
'It doesn't need batteries either.' she continued.
'Wow! This is so cool!' they exclaimed. 'Someone should have invented this a long time ago!'