Consider donating blood, especially if your blood type is "0".

Donor's (D) blood type that contains either "A", "B" or "+" can be given only to recipient (R) whose blood type contains these symbols. Thus "AB+" type patient will accept all blood types, while "0-" patient needs exactly "0-" donor (7% of population).

D ∪ R = R, where D, R ⊆ {"A", "B", "+"}

From breadcrumbs collected so far we have:
(1) Physical fields of space
(2) through which light travels at constant speed and nothing can move faster,
(3) but in a moving frame of reference photons bounce back from mirrors and come back at the same time regardless of direction.

Animation below shows that it would work if lengths were contracted in the direction of motion, but we assumed they're equal. How do we measure distance anyway?

#SpacetimeBreadcrumbs

This is simplified version of expected result from Michelson-Morley experiment. We know that this is not what happened and photons came back (to A) at the same time.

Note that photons didin't move at the same speed relative to the central object A (see previous entry). That's not how Special Theory of Relativity (STR) solved this problem though.

#SpacetimeBreadcrumbs