The grandson got another cup of tea and set it down once again on the furniture, the Star Tribune reported. Washington pulled out the .38 Special and shot him, police said.
KSTP in St. Paul reported that Washington told investigators she was angry because her grandson had put his cup on a wooden table.
Investigators found the revolver at the scene, the Star Tribune reported. It was loaded with five rounds, one of which had been spent.
A judge has ordered Washington to undergo a mental evaluation to see if she is competent to stand trial, the newspaper said.