If an apple a day keeps the doctor away, then apple is medical word. Tsuwm's wwftd "etaerio"
I had never heard before, though I have eaten many of them. Here's a paragraph about them,
with several technical words to learn:

"Apples (Malus communis, M. pumila, & M. sylvestris), pears (Pyrus communis) and quince
(Cydonia oblonga) belong to the rose family (Rosaceae), and include literally hundreds of cultivated
varieties. In the apple, the original ancestral species is obscured by so many cultivated variations
throughout the centuries that some authors lump them all into one species, Malus domestica. They all
originated in western Asia (or Eurasia) and are characterized by fleshy fruits called pomes. In the pome, a
thick, fleshy hypanthium layer (also called the floral cup or calyx tube) surrounds (and is fused with) the
seed-bearing ovary or core. The sepals, petals and stamens arise from the rim of the hypanthium. Since
the ovary is situated below the attachment of the sepals, petals and stamens, it is termed "inferior" in
technical plant taxonomy books. The fleshy hypanthium of a rose (Rosa) surrounds a cluster of small
one-seeded achenes. Since the achenes represent separate ripened ovaries all derived from a single flower,
the entire structure (called a rose hip) can be considered an aggregate fruit or etaerio. Rose hips are eaten
raw and are ground up as a supplemental source of vitamin C (ascorbic acid).

-tsuwm http:///home.mn.rr.com/wwftd/

Geometrical question: how can you move an apple in such a way it seems not moved?

(in technical words: which are the isometries or rigid movements preserving an apple)

In other words , it seems that you can rotate an apple around the axis , in any way...
but looking better, and splitting the apple by a cut perpendicular to the axis, you see the five seeds, so that you can rotate the apple just of 72 degrees, 144 degrees... and so on

More difficult to explain by words than to show using a true apple. Try!

Do I understand you to mean, Emanuela, that, should you cut the apple so that the "star" of seeds appears through the center, you can then rotate it so as not to be able to tell that the apple has been rotated, distintive features of individual seeds not taken into consideration?

Another way of expressing it is that the apple has "five-way symmetry" on the inside, though from the outside it appears to be symmetric to any degree of rotation.