Two "stupid" and "bored" workers set fire to a mouse that led to a blaze in a cycle shop.
The fire at Cycle King Shop in Bury St Edmunds, Suffolk, caused £1.6m of damage, Ipswich Crown Court heard.
Ashley Finley, 25, of Elmswell, and Dysney Sibbons, 23, of Stowmarket, admitted arson by reckless behaviour on the first day of their trial.
Two neighbouring buildings were also damaged in the fire on Angel Hill. The pair will be sentenced next month.
Judge David Pugh said the fire was an "act of sheer stupidity" by the two defendants, who had tried to "alleviate what they appeared to have found a boring day by cremating a mouse".
The court heard some form of accelerant was used in the deliberate act on 29 September 2017.
Finley, of Borley Crescent, and Sibbons, of Elmsett Close, have been released on bail pending sentencing.