A Little Logic Puzzle

Consider the following syllogism.

Premise 1: All living things need water.
Premise 2: Roses need water.
Therefore: Roses are living things.

Is the conclusion logically valid or invalid? Try to answer before reading on.

About 70 percent of university students who have been given this problem think that the conclusion is valid. They were wrong, probably by confusing truth and logical validity. The syllogism's form is as follows.

Premise 1: All S is P.
Premise 2: Q is P.
Therefore: Q is S.

This is more likely judged as invalid, since true/false doesn't interfere.

The following with the same form is more obviously invalid.

Premise 1: All cows breathe air.
Premise 2: My cat breathes air.
Therefore: My cat is a cow.

If the conclusion and Premise 2 of any syllogism above were reversed, it would be logically valid. Of course, 'my cat is a cow' would remain false.