Analogies

Sometimes analogies help to explain this to people new to the area.

Minefield

Richard Stallman has frequently mentioned this analogy in parts of his speeches:

• http://www.gnu.org/philosophy/stallman-mec-india.html
• http://www.gnu.org/philosophy/fighting-software-patents.html
• http://www.ifso.ie/documents/rms-2004-05-24.html

This analogy omits one aspect: when you step on a mine, you know it, but when you violate a patent, the patent holder might threaten you immediately, or might decide to let you continue to build you project on that idea and then threaten you.

Literary patents

Again citing Richard Stallman:

• http://www.gnu.org/philosophy/software-literary-patents.html