Simplify the following text for a Grade 10 native English-speaking person.
Original: Planck units are only one system of natural units among other systems , but are considered unique in that these units are not based on properties of any prototype object , or particle ( that would be arbitrarily chosen ) but are based only on properties of free space .
Simplified: Planck units are only one system of natural units among other systems . They are considered unique , because these units are not based on properties of any prototype object , or particle ( that would be arbitrarily chosen ) but are based only on properties of free space .