Crazy Calculator is a DIY wooden model kit based on an additor that was invented by a St. Petersburg music teacher, Heinrich Kummer, in 1846. Its simple design and compact size made it a popular mathematical tool in its time.

It is used to add and subtract numbers. To do this, the user enters a number value using the stylus, then uses the stylus to enter a second number. The device then shows the result of the desired mathematical operation, according to where the second number was entered (top row for addition, bottom row for subtraction). But this Calculator is not limited to addition and subtraction—you can also use it to multiply and divide.