The click functionality can be implemented using Checkbox for pure css. I modified your HTML as follows:
HTML
some text...
CSS
:checked ~ .btn-default, #show, .checkbox {
display: none;
}
:checked ~ #show {
display: block;
}
.show-text:after {
content:"Open";
}
:checked + .show-text:after {
content:"";
}
.second-label, .show-text {
text-decoration: underline;
cursor: pointer;
}
Working Fiddle