In response to:

Lectures in Higher Mathematics. The teacher writes on the board a cracked formula and turning to the hall says:
The figure drawn according to this formula will look like a sand clock.
From the hall:
What form?
Teacher without a smile:
Hyperbolic parabolic
From the hall:
Now it understands.
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The hyperbolic parabolic has the shape of a saddle, smart people! And the sand clock is a single-band hyperboloid!!! to