Determining validity of an argument I would like to know whether the following argument is valid. 
Some amphibians live in the water
All fish live in the water
Therefore, some fish are amphibians.
 A: Venn diagrams are sometimes helpful in seeing what’s going on. The conclusion Some fish are amphibians would fit this diagram:

However, the hypothesis fit this diagram just as well, and it describes a world in which no fish are amphibians:

Since the second diagram is consistent with the hypotheses and contradicts the conclusion, the argument cannot be valid.
A: This argument is not valid.  Notice that some violins make sounds and all pianos make sounds, but this does not mean that some pianos are violins.
The argument is essentially
$\exists x ( \operatorname{Amphibian} (x) \land \operatorname{Water} (x) )$
$\forall x ( \operatorname{Fish} (x) \implies \operatorname{Water} (x) )$
$\therefore \exists x ( \operatorname{Fish} (x) \land \operatorname{Amphibian} (x) )$
The argument would be valid IF the second statement were changed to:
$\forall x ( \operatorname{Water} (x) \implies \operatorname{Fish} (x) )$  (All things that live in water are fish)
In its current form, the argument is an example of affirming the consequent.
