Why Not Teethpaste and Teethbrush?

I thought of resuming my Posts.

I am trying to remember events which happened during my bed rest time.

Event 1:

Abhinav was learning Singular and Plural. I was telling him as Eye & eyes, Ear & ears….

He asked Plural for Teeth. 

I said “ singular is Tooth and Plural is Teeth”.

In no time, He said , then why we are telling toothpaste, it should be teeth paste, teeth brush?

Because we are using for all tooth not one.

I was blinking and said” English is funny Language”. Actually I don't know.

Event 2:

Abhinav Knows about Six Fingers hand from his previous observation.

He was asking me yesterday night that “ What you call for Last small finger in Hand?”. 

I Said “ It is Little finger” and You know it Abhinav.

Then he asked what you call for Little finger if somebody has another Little finger (6^th Finger)?

I was blinking as usual, and I need to learn Medical terms to answer that question….