Regarding the cap diameter, it is typical that
bottle caps are cylindrical. Experiments confirmed
that the evaluation of the cap diameter
efficacy is based on the ability to tightly
clamp the cap with the thumb (from the first
segment beginning with the tip) and index finger
(to the second segment). Effective grabbing
and clamping by the average child is based on
the use of the right hand: the curved length
from the first segment of the thumb beginning
with its tip to the second segment of the
index finger. The mean value is approximately
one-half of the circumference of the cap.However,
to prevent 97.5% of the children from
tightly grabbing, clamping, or conveniently applying
force to a twist to open the cap, the
value of the cap diameter was calculated as
follows: