function openCaja() {

Effect.BlindDown('caja');
Effect.toggle('capa_s', 'appear', {duration: 1, from: 0.0, to: 0.5});
}

function cierraCaja() {
Effect.BlindUp('caja');
Effect.toggle('capa_s', 'appear', {duration: 1, from: 0.5, to: 0.0});
}

Event.observe(document, 'click', revisarClick);

function revisarClick(event) {
var element = event.element();
if (element.id=='capa_s') cierraCaja();
if (element.id=='e_abrir') openCaja();

}