What is the greatest common factor of 40 and 36

Nobody goes around with lists of greatest common factors in their head. 
When we need to know one, we grab the nearest pencil and work it out.

The factors of  40  are  1,  2,  4,  5,  8,  10,  20, and  40 .

The factors of  36  are  1,  2,  3,  4,  6,  9,  12,  18, and  36 .

The common factors of  35  and  40  are  1,  2, and  4 .

The biggest one is  4 .