How Big Is a Gallon?

Had a customer come into the store today who had had a couple of goldfish in a small tank with a pleco, but the fish had gotten sick and so they upgraded to a larger tank...but the goldfish still died. So the customer was wanting to know what would fit into their tank with a pleco. 

Which begged the question of 'how big' the tank was. 
Because the would influence the type of fish that I would recommend.

The customer told me that they had upgraded to a 17 gallon tank.

Which...was a weird size...but I suppose it was something that could be made.

But at some point...after we were talking about heaters for fish because if they wanted something besides goldfish they would need to do that....
I had her show me the tank they had 'bought here' because...we don't sell 17 gallon tanks.

So after some deliberation and pointing out different sized tanks...
The customer realized that the tank that had been bought (not by them but by their spouse)

… Was actually a Three point Five Gallon Tank. 

*shakes head* 3.5 gallons is a huge difference from 17 gallons.

And the customer was shocked to realize that the tank wasn't as 'big' as they thought it was. 
which had them wondering how big their original tank was...probably a gallon maybe is my guess.

But yah. 

17 gallons....

Makes me wonder if they comprehend how...much a gallon is....Like....*shakes head* 
I don't get it.
Don't get it. 

-Sarnic Dirchi