A method can include applying a first voltage to a first memory cell to activate the first memory cell, applying a second voltage to a second memory cell coupled in series with the first memory cell to activate the second memory cell so that current flows through the first and second memory cells, and generating an output responsive to the current. The first voltage and a threshold voltage of the second memory cell can be such that the current is proportional to a product of the first voltage and the threshold voltage of the second memory cell.