A lighthouse is the classic example of a public good because it is difficult to prevent a ship (a free rider) from using it.

Observe the relative position, returned the Doctor with a smile. It is your attitude to believe through thick and thin in one man's judgment—your own. I follow the same opinion, but critically and with open eyes. Which is the more irrational—I leave it to yourself. Oh, my dear fellow! cried Casimir, […] don't ratiocinate with me.

The 5-nitroimidazole molecules are very potent anaerobicidal agents commonly used to treat or prevent Bacteroides infections.

That subject has already been fully exhausted.