A grandad claimed he lit a flare before an Old Firm game to show how it worked to his autistic grandson.
Archibald Smith, 58, sparked the pyrotechnic close to Celtic Park on February 2, 2022.
The Hoops were due to host rivals Rangers in the Scottish Premiership evening fixture.
Smith had earlier attempted to stamp on the flare to put it out when caught by police.
However, Sheriff Lindsay Wood told him today that his excuse for committing the crime was "a lot of nonsense”.
Smith pleaded guilty at Glasgow Sheriff Court to culpable and reckless conduct.
The court heard it was 7pm when Smith was on Springfield Road surrounded by approximately 100 people.
Prosecutor Heather Naismith said: "Police saw Smith holding a green pyrotechnic device in his hand [and] thereafter light the flare.
"A plume of green smoke emerged from the device as he raised the flare to shoulder level.
"Police approached him and he thereafter dropped the flare on the floor and attempted to extinguish it by stamping on it."
Smith was later arrested and missed the match which Celtic went on to win 3-0.
Smith's lawyer told the court that he went to the game with his 13-year-old grandson who was handed a flare by an unknown man.
Marc Waters added: "His grandson is diagnosed with autism and he took it from him to show what the flare does."
Sheriff Wood replied: "What a lot of nonsense."
Mr Waters responded: "I know that."
Sheriff Wood asked Smith for "the truth" and he stated that he got "caught up in the occasion”.
Sentence was deferred pending background reports until next month.
Sheriff Wood told the grandad of five: "A football banning order will be coming your way."
Smith, of the city's Carntyne, had his bail continued meantime.