I sprinkled a little crushed cat food on my mother-in-law's cupcake
Why would you do that? Did she notice the cat food?
She didn't notice anything strange at all.
I'm sure you were just playing a prank, but that still doesn't make it okay. What you did was mean and you shouldn't do things like that to your in-laws. I hope she doesn't get sick from it!
I was testing to see if she was allergic to cat food.
And what if she was allergic? I'm sure you'd feel badly if you'd hurt your mother in law wih your mean prank.