If it's not yours don't damage it!
Treat it as if it is yours
Don't break it!
Would you want someone to show disrespect to one of your expensive things? Well if you don't want that to happen, don't show disrespect to something if it is not yours!
It will cost you!
If you damage something that it is not yours it will cost you, since it was your fault and you was not showing respect to it. I don't think anyone would want to spend their money on fixing something they broke on purpose
Respect
If it is not yours you have got to show respect, because you will end up feeling guilty for damaging something that is not yours. Also, you will upset the person who owns the thing you damaged so, please don't do it.