" Gramps saw a car pull out of a space

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?

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

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

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

The narrator knows that Gramps was joking because of a few clues in the text. First, when the man in the other car waved his fist at Gramps, instead of responding seriously or getting upset, Gramps immediately claims to be a veteran and shows his leg, implying that he is using his veteran status as an excuse for his actions. This exaggerated reaction suggests that Gramps is trying to make a joke or play a prank.

Second, Gramps writes a long note about being a World War Veteran with German shrapnel in his leg, and he puts this note on the dashboard of the car. This note serves as a punchline to his joke, as he exaggerates his story to justify not having the correct change for the parking meter. This further confirms that Gramps is making up the story about his imaginary shrapnel.

Lastly, when the narrator directly asks Gramps about the shrapnel in his leg, Gramps avoids answering the question directly by looking up at the sky and commenting on the weather. This diversion suggests that Gramps is intentionally avoiding giving a straightforward answer, reinforcing the idea that he is joking about having shrapnel in his leg.

By considering these clues, the narrator realizes that Gramps is joking and that the story about the German shrapnel in his leg is not true.