I'm kidding!
A weak excuse you use to avoid being serious, like a kid who doesn't want to do homework.
'I'm kidding!' he said, but I knew he wasn't because he didn't want to do homework.
She said, 'I'm kidding!' but I knew she wasn't because she didn't want to do homework.
I said, 'I'm kidding!' but I knew I wasn't because I didn't want to do homework.