Inside (or "female") condoms work the same way outside (or "male") condoms do: by containing semen, and the sperm cells it contains, inside the condom.
A condom still surrounds the penis -- and in fact, covers more surface area than outside condoms do -- it's just tethered to the vagina and vulva instead of to the penis. Make sense?
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I'm not quite sure what you mean. Around the area of the vulva there are three openings: the vaginal canal, the urethra (this one is very small and harder to spot) and the anus - there's a good diagram and explanation here which you can look at for more information: Innies & Outies: The Vagina, Clitoris, Uterus and More.
Internal condoms are placed inside the vagina before vaginal intercourse - so you just need one at a time (though you should use a new condom every time you have sex!).
Sorry Siân. What I meant was that doesn't the female condom have two openings - two holes - one on either side? Wouldn't the semen fall straight through?
Ah, thanks for clarifying! Nope, the internal condom only has one opening, the same as an external (male) condom does; the opening that the penis goes into. There's actually an illustration of it, and a diagram on how it's used, in this article: All the Barriers! All the Time!
Internal condoms do not come in different sizes. One of the benefits of internal condoms is that they are able to take the shape of the vaginal canal once they are inserted & warm up from body heat, so they are sort of one-size-fits-all.
I was just about to jump in with an answer but I see everyone else got there! Glad it was helpful GS.
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