It's hard to be Sheldon Cooper at any age.

The Big Bang Theory gave its characters a lot of things to be characterized with. Raj was the shy one, Howard the sleazy one (at least for the first seasons), Leonard was the most insecure man in the world. And only Sheldon was so many things at once that it's hard to count and to make a top list.

And when the prequel spinoff Young Sheldon was released, it finally cleared out some of his character's traits. Now let's have a look at what else Young Sheldon mentioned from the original show.

1. Professor Proton

The Big Bang Theory introduces Arthur Jeffries a.

k.a. Professor Proton, one of Sheldon's beloved people. He is actually the man who inspired him to become a scientist. And Young Sheldon didn't miss on that, too.

Bob Newhart made a cameo appearance in the show's pilot, as Sheldon was watching his show on TV. Later he appeared two more times, and then was referred only verbally.

2. Mary's smoking

As Sheldon once revealed in season 2 of The Big Bang Theory, his mother has a small smoking habit. "Jesus is okay with it, but we can't tell Dad." And really, in Young Sheldon, we see Mary with an occasional cigarette when she feels tense.

This habit of hers may actually be all that is left of her rebellious teenage past.

3. Tutoring Experiments

We know for a fact that Sheldon is not the best example when it comes to basic human ethics. But these two times he tried to be a tutor definitely rank among his worst behaviors.

In The Big Bang Theory season 3, Penny asked him to teach her "a little physics," and Sheldon created a so-called "Project Gorilla." But as it turns out in Young Sheldon, he already tried something similar with his sister Missy, although this time he chose the approach based on teaching monkeys.

4. Soft Kitty...

One of the sweetest scenes in The Big Bang Theory is the one Penny and Sheldon share when he is sick. He tells Penny that there is a song his mother used to sing to him and asks Penny to do the same.

In Young Sheldon, we actually witness Mary singing "Soft Kitty" to Sheldon on his bed when he has the flu, at the end of episode 13 of the season 1.