Think about your feet in wet grass or on the hot sand of a beach. A sharp needle prick or the shooting pain when you bang your elbow. Without a sense of touch, all those experiences would feel pretty much the same.

Thousands of receptors for touch work together to tell the brain about the outside world. A harmful touch is identified by specialised receptors as a source of pain. Whether a touch is good or bad, a signal travels from sensors in the skin through nerves to the spinal cord and the brain, where it is turned into the feelings we recognise.

You don't always notice touch - you soon stop paying attention to how your clothes feel once you've put them on - but there are some touches we just can't ignore, like being tickled.

Sarah Blakemore does research into tickling and asks questions like, "Why can't you tickle yourself?" She thinks that because the brain also knows what the fingers are doing, that information overrides the tickle sensation. You can watch Sarah Blakemore's tickle experiment from the programme, showing that no matter how hard you try, you can't help but laugh when you're tickled by someone.

Surprisingly, surprise doesn't matter when you tickle your victim. If you threaten to tickle someone and then do, they'll laugh just the same as if you caught them unawares. Brain scans of people being tickled show that the way the brain reacts doesn't change for so-called anticipated tickles. Bet you never thought tickling was such a science.

What if you had no sense of touch? You don't just miss out on experiencing textures, it actually makes it very difficult to move. How do you walk if you can't tell when your feet are on the ground? The programme meets Ian Waterman, who lost his sense of touch at the age of 19. You can find out more about how he's dealt with his condition at www.apa.org/monitor/jun98/touch.html. Ian's story was the subject of a BBC Horizon film in 1998.