As you probably know, babies have several reflexes, some of which they are born with - Moro (startle), walking, rooting (searching for food i.e. the nipple) and some they develop later.
This reflex of sticking the tongue out is one that develops later, usually about 3-4 months. It is also known as the extrusion reflex. See it listed amongst other key ones here: http://nursingcrib.com/nursing-notes-reviewer/maternal-child-health/newborn-reflexes/
You will know if it is present because if you try and give solid food it ends up being thrust out by the tongue as fast as you are trying to spoon it in!!! See this YouTube clip for a great demo of this. The baby rice on the spoon is a little too thin so that will make it more difficult.
So if your little bubba is displaying the extrusion reflex don't worry, it's normal. It's just their way of saying, 'I'm not quite ready for food yet."
So just wait a week or two it'll be gone and then you're ready for solids.
Another reason babies stick their tongue out especially from 2-4 months is that they are tired and need sleep. It's one of the many tired signs a baby shows. Others include jerky body movements, averting their gaze, crying or cranky behaviour and yawning. You need to see several tired signs before your baby is ready for sleep.
If you'd like to know more about this it's all in the explanatory videos in my NEW Nurture Sleep Program.