F is for friction.... the coefficient of friction to be precise.
According to wiki:
"The coefficient of friction (COF), often symbolized by the Greek letter µ, is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction, while rubber on pavement has a high coefficient of friction. Coefficients of friction range from near zero to greater than one.."
In this case, we are referring to the surface of my living room floor laminate, and some blackberries.
My toddler was on the floor next to me in the living room. I could see her face and chest. I could not see her feet and hands. She was watching tv and going- weeee weee weee and moving about. I was so pleased. She was having a good time and I was able to pay some bills online.
Next thing, I notice she has her snack with her. Her bowl of blackberries. Her ears have something... wait.. her hands.. ummm.. OMGGGG
She rubbed blackberries all over herself, and was skating on the blackberries smashed under her feet.
Apparently the blackberries and laminate have just the right coefficient of friction to please a toddler.
I wish I was less afraid of parades of ants, and had taken some pictures before I cleaned it up.. but I just had to clean it. And her.
And yea, blackberries stain.