A specially enchanted egg of this sort is always coated with oil of timelessness. 

Although very old and quite vile-smelling, each such egg has the possibility of increasing the eater's Intelligence or Wisdom. 

However, an individual who consumes an egg of reason must first make a saving throw versus Poison—with no bonuses whatsoever—or else the matter consumed will slay him or her.

The effect of this poison cannot be neutralized. 

If eating the egg does not prove fatal, there is a possibility that the consumer will gain one point of either Intelligence (60% chance) or Wisdom (40%). 

The chance of successful enhancement is equal to the eater's current ability score (whichever applies), doubled and subtracted from 100.

Example: An egg which improves Intelligence is eaten by a character with an Intelligence score of 17. The chance of successful enhancement is 100 - (17 x 2) = 68%. Ability scores of 18 or higher are effectively enhanced by only 1/10 point at a time (cf. DMG p. 11, "The Effect of Wishes on Character Ability Scores"), so that a character would have to successfully consume 10 eggs of reason (if, somehow, that many were available) in order to raise an ability score from 18 to 19.

Racial limitations apply when determining how much an ability score can be enhanced. 

If the oil of timelessness is removed from an egg of reason before it is consumed, the egg will still have a chance of poisoning the eater but will have lost its ability-enhancing property.