Well, the difficulty in explaining the fourth dimension is that it cannot be seen or felt.
5 seconds sound clip from the The Time Machine (1960) movie soundboard.
You can hear this line at 00:07:54 in the Blu-ray version of the movie.
Quote context
[...]
- That's very clever of you, indeed.
- I don't think George is referring to a new type of timepiece.
- When I speak of time, gentlemen, I'm referring to the fourth dimension.
- Well, the difficulty in explaining the fourth dimension is that it cannot be seen or felt.
- If you don't mind, will you refresh me on the first three dimensions?
- Really, Filby. Surely they taught you something in school.
- Suppose you explain it, Doctor.
- For example, when I move in a straight line, forward or backward, that's one dimension.
[...]