Example from Charles Travis:
- Maple trees have red leaves naturally, but Pia paints them green.
- One day, a decorator asks her if she has any green leaves for her display, and Pia answers yes.
- Another day, a chemist asks her if she has any green leaves for her experiment on green leaf chemistry, and Pia answers no.
- In both cases, Pia answered truthfully.
The moral: any true sentence in some context has a context which would make its negation true.