function hideActivity() {
  document.getElementById('post-container').style.display = 'none';
  document.getElementById('photo').style.display = 'none';
  document.getElementById('link').style.display = 'none';
  document.getElementById('music').style.display = 'none';
  document.getElementById('video').style.display = 'none';
}
 
function showActivity(id) {
  hideActivity();
  document.getElementById('post-container').style.display = 'block';
  document.getElementById(id).style.display = 'block';
}

function blurActivity(obj) {
  if(obj.value == '') {
    obj.value = 'Napisz cokolwiek...';
    obj.style.color = '#ccc'	
  }	
}

function focusActivity(obj) {
  if(obj.value == 'Napisz cokolwiek...')
    obj.value = '';
  obj.style.color = '#000'	
}