They then return the packages to get a refund.
The tampered boxes turned up in four stores in Suffolk and Nassau counties.
Investigators are calling it a form of shoplifting.
Suffolk County authorities said at least 10 tampered packages of Jell-O brand pudding mix had been discovered on March 24 and March 25.
The tampered boxes were found in three stores in Suffolk County - Setauket, Smithtown and Huntington - and one in Nassau County.