In some elementary courses, it is shown that square root of 2 is irrational. It is also shown that the roots like square root of 3, cube root of 2, etc., are irrational. Much less often, it is shown that the number "e," the base of the natural logarithm, is irrational, even though a proof is available that uses only elementary calculus. In this short note, the authors repeat this proof and find a collection of other numbers that can be proved to be irrational in a similar way. While this work is not new, the presentation here is simpler than others. It is appropriate for students in calculus, advanced calculus and real analysis.