A dam is a small square of soft plastic which makes performing oral sex a whole lot safer. It sits over the female genitals or anus - kind of like a safety blanket - so that oral sex can be performed with less risk of catching STIs.
While dams may not be commonly used, that's not to say they are unimportant. A new study revealed men who smoke and perform oral sex with five or more partners have the highest risk of developing a type of head and neck cancer triggered by the human papilloma virus (HPV).
Dr Peter Greenhouse, a sexual health adviser for the government, has also warned that more and more people are contracting sexually transmitted infections (STIs) through oral sex. And dams are just one way of preventing this.
To raise awareness of the mighty yet little-known dam, we asked sexual health charity FPA all of the questions you're too afraid to.
What is a dam?
A dam is a latex or polyurethane (soft plastic) square or rectangle, which can be used to cover the anus or female genitals. It acts as a barrier to help prevent sexually transmitted infections passing from one person to another.
They're not suitable for use during penetrative sex and for oral sex involving a penis and mouth, a condom can be used.
Where can you get them from?
You can get dams at some genitourinary (GUM) and sexual health clinics, contraception clinics and pharmacies. Don't be too surprised if your local clinic doesn't hand them out though, as they may not get asked for them very often.
They are also widely available from online shops which sell condoms.
How does it stay in place?
To keep a dam in place during oral sex you simply hold it with your fingers. It doesn't matter which side you put against the vulva or anus, but you shouldn't turn the dam over once you've started using it.
You also shouldn't move a dam from the anus to the vulva because bacteria that live harmlessly in the anus can cause infection in the vagina.
Does public hair matter?
Pubic hair doesn't matter too much, though there may be some rearranging to be done to enable the dam to directly touch the vulva and allow stimulation from lips and the tongue.
Remember the dam can only protect the area it is covering, so, for example if someone with pubic lice was receiving oral sex, it would be possible for this to be passed on to a partner if there is hair to hair contact beyond the edges of the dam.
If you stick to the middle of the dam, it should be big enough to provide protection. If you've fashioned a condom or latex glove into a dam, just be careful if the surface area isn't quite as big as a regular dam would be.
What do dams protect against?
In the same way as condoms, dams form a physical barrier to prevent STIs (bacterial, viral and parasitic) from being passed from one person to another.
Generally the risk of STIs associated with oral sex is less than vaginal or anal sex, but some STIs are more easily passed on during oral sex than others. These include Herpes simplex (which causes genital herpes), gonorrhoea and syphilis.
Why aren't they offered as much as condoms?
Dams possibly aren't offered as much as condoms because people don't ask for them as much, and perhaps because the risk of STIs with oral sex is not as well known or commonly talked about.
However we know that oral sex is very popular. In one of the biggest studies into sexual behaviour in Britain, 70% of 16-24-year-old women and 80% of 25 to 34-year-old women.
Among men, figures were almost the same:71% of 16 to 24-year-olds and 80% of 25 to 34-year olds said they'd given or received oral sex in the last year.
In the FPA Sexual Health Week survey, most people said they hadn't learned about the risk of STIs and oral sex when they were at school, and the majority said they didn't know STIs could be passed on during oral sex.
How much do they cost?
Bought on their own, dams are around £1 to £1.50, depending on the kind, or it's possible to get multipacks more cheaply.
They are also free from some clinics.
Can they break?
A dam shouldn't break just from being used as a protective barrier during oral sex. However, some can be damaged by lubricants, depending on the material.
You can use water-based lubricants with latex dams, and it is safe to use oil-based lubricants with polyurethane dams.
Using lube can help make receiving oral sex more pleasurable.
What flavours are there?
Dams come in a whole range of flavours including chocolate, blueberry, mint (tingly), banana, vanilla, grape and cola - to name a few.
What do they feel like?
In a similar way to condoms, dams won't make the user feel like they are directly touching a partner's genital area with their lips or tongue, and vice versa.
But they are thin enough to enable pleasure and stimulation, which means you can still enjoy great oral sex.
Knowing you're protected from STIs can actually make oral sex feel even better as you can relax a bit more.