I have asked this same question and it was closed in less than two minutes, I really don't understand why is it very stupid? (sorry for being very stupid).
I've come across the following exercise:
Prove that the solid torus denoted $\tau$ which is obtained by rotating the disc $\lbrace(x-1)^2 +z^2 \leq \frac{1}{4}, y= 0 \rbrace$ around the axis $\lbrace x=y=0\rbrace$. Prove that $\tau$ is diffeomorphic to $D^2 \times S^1.$
I couldn't start thinking of this exercise since I didn't understand the description of the solid torus as it is mentioned in the exercise, I hope that some could give more explicit explanation of what it is please.
I am self studying, and any comment would so much appreciated!
$\textbf{Edit}$: what I mean by explicit explanation is as follows: for example if I take a point $z$ in the disc $D^2$ or in the circle $S^1$ I know that it satisfies $\vert z\vert \leq 1 $ or $\vert z \vert = 1$ respectively. However if I take a point in the solid torus I don't know which equation it satisfies ?
