Why can I squirt a jet of saliva?

Matt - That's very fascinating! Thanks, Chris. Finally closing the show, Hannah's got the answer to this week's disgusting Question of the Week...

Hannah - This week, we sleekly squirt into a question that Simon Ashby wrote in with...

Simon - In the base of my mouth below my tongue, I am sometimes able to eject a very fine spray of saliva out of my mouth. I know I'm not the only person able to do that as a friend of mine could do this at will. He would lift and move his tongue and produce a fine jet of saliva that's squirted from his mouth. The question is, why are we evolved to be able to do this?

Hannah - It's called gleeking and it's pretty gross, but can you do it? If so, why do you think it happens? To facilitate social bonding perhaps? To find out, we speak to Professor Gordon Proctor from the Salivary Research Unit at King's College London.

Gordon - Hello. I've come across the occasional gleeker. Most saliva is made and secreted by the major salivary glands. If you lift your tongue and look in the mirror, you can see the swellings or papillae which are the ends of the tubes. That's the ducts that take saliva from one pair of submandibular salivary glands to the mouth. When we taste food, our salivary glands greatly increase the production and secretion of saliva. I think that you can gleek more effectively after having tasted foods that have a strong stimulus for salivation - for example, lemon sweets. So, gleeking is probably achieved by the compression of the ducts of the submandibular glands by muscles in the floor of the mouth when you move your tongue upwards. It may be that in gleekers, a large volume of saliva builds up in the ducts before being expelled.

Hannah - Thanks, Gordon. So, gleeking is possible as salivary glands have evolved to squirt saliva from the mouth. Human saliva is 99.5% water and the rest is electrolytes, mucus, glycoproteins, enzymes, and antibacterial compounds. These help digest and lubricate food and prevent tooth decay. Usually, we keep it inside our mouths, but gleekers take advantage of spit to project it from their submandibular gland, out into the world. Gross! Now, Juliet doesn't spit in her garden or anywhere. She does sometimes loose her glasses there though...

Juliet - Hi. Today in the garden, I was multi-tasking a bit too much and part of this involved putting my glasses on top of my head. Naturally, they fell off. I looked everywhere, but couldn't see them even though they are bright purple and the garden is mulched in light tanned straw. I found them close by where I had surely looked a minute before. Why didn't I see them the first time my eyes passed over that spot?

Hannah - So, why is it that we sometimes overlook the obvious? Why could Juliet not find her glasses? She swears she's not blind without them, so what happened? What do you think?

Chris - Hannah Critchlow.