Friction

Diesel v petrol!

Ok, bits ripping molecules off each other. Cured by adding oil film at least in an engine!

Lumps.

A

Me coming home late with lipstick on my collar

1.. The rubbing of one object or surface against another. 2.. Conflict, as between persons having dissimilar ideas or interests; clash. 3.. A force that resists the relativ motion or tendency to such motion of two bodies in contact.

Telling your girlfriend you want to shag her mates.

No, its when you do it the other way around!

Or telling her that you didn't, honest, she's lying about it. :)