Employee was getting a piece of sheet steel out of the steel rack. The employee needed a 1/8"" x 4 x 8 it was in the middle of 3/8 x 4 x 8. They leaned the 3/8"" sheets to access the 1/8"". Employee went to retrieve 1/8"" sheet the 3/8"" sheets fell back upon the 1/8"" striking employee's right thumb.