One type of minimum-maximum thermometer consists of a U-shaped glass tube, which is designed to read the correct temperature at both ends of the enclosed column of liquid. There is a (metal) block at each end of the liquid column that is pushed along as the column advances. When the column retreats, the block stays behind - marking the highest and/or lowest temperature. These metal blocks can then be reset by pulling it back to the end of the liquid column using an accompanying magnet. There are digital thermometers that remember high and low values. The "analog" Min-Max thermometers have a small marker that gets pushed into place by the liquid in the thermometer (mercury or an alcohol mixture). These remain in place even after the liquid has expanded (for minimum range) or contracted (for maximum range).