You do not wash the cup, you wash the dirt; if you were to wash the cup, it would disappear completely. So, it is the dirt that is washable and has nothing at all to do with the cup.
This example is quite a good illustration of one of the most profound theories of the bodhisattvayana: we all have the potential to become Buddhas because we have Buddha nature. The problem is we have yet to realise it.

Dzongsar Khyentse Rinpoche
from the book Not for Happiness: A Guide to the So-Called Preliminary Practices
Read a random quote or see all quotes by Dzongsar Khyentse Rinpoche.
Further quotes from the book Not for Happiness:
- Life is a stream of sensory illusions
- What is merit
- Avoid being distracted
- Absolutely nothing genuinely works
- Self Trapped
- Remain alone and practise the dharma
- Not designed to cheer you up
- Sign of a mature practitioner
- No end to samsara’s sufferings
- Intention to benefit all sentient beings
- Wishing happiness for those who have hurt you
- Filtered perception
- Being able to start practicing right away
- Sources of our inspiration
- Rip that ego apart
- Relative and absolute truth
- It’s all a matter of motivation
- The decision to follow a spiritual path
- Three higher trainings
- Opposite direction to dharma