" Gramps saw a car pull out of a space

Question

along one of the main roads Another car saw it too, but Gramps was fast and
pulled in, and when the man in the other car waved his fist at Gramps,
Gramps said, 'I'm a Veteran. See this leg? Shrapnel from German guns. I
saved our country!' The man stared at Gramps.
We did not have the correct change for the parking meter, so Gramps
wrote a long note about how he was a visitor from Bybanks, Kentucky, and
he was a World War Veteran with German shrapnel in his leg, and he kindly
appreciated the members of the fair city of Madison allowing him to park in
this space even though he did not have the correct change for the meter. He
put this note on the dashboard.
'Is it true?' I asked him. 'Do you really have German shrapnel in your leg?'
Gramps looked up at the sky. 'Mighty nice day. he said.
The shrapnel was imaginary. Sometimes I am a little slow to figure these
things out. "
What makes the narrator know that Gramps was joking?

Answer 1

I think because Gramps 'ignored' the question, but I'm not sure.

Answer 2

Because he would not confirm what he had told the strangers when his grand child asked him.

Answer 3

Yes, that's where I was getting at too. Thanks.