What is a 3D pentagon & Octagon called?

[Tidgypuds]

Tried googling it and I think a 3D Pentagon is a decahedron, is this right?

What is the 3D Octagon called?

This is my DS (7) asking? He was just stalling so he didnt have to go to sleep so I promised I would find out for the morning.

dodecahedron is the pentagon.

octahedron??

I think, although I'm not sure, that some shapes won't form a 3-d shape because they don't tesselate correctly, so you'll end up with a 3d shape made up of octagons plus squares or triangles.
You could give him 4 equalateral triangles and get hin to make a 3-d shape - easy

and then a pile of pentagons and see what he can come up with,

[[http://www.korthalsaltes.com/foto/octagonal_prism.jpg octahedron is something else]

You can't make a 3D shape just using octagons, in the way you can make an icosahedron with triangles and a dodecahedron with pentagons.

You can make a shape using octagons and small squares ... but I can't remember what it's called!

try that link again

Is octahedron a real word then? I just made it up

Are you thinking of a pentagonal prism and and an octagonal prism?

pentagonal prism

octagonal prism.

I don't think7yos would be doing dodecahedronsa nd such but they would be doing prisms.

Thanks for those.
Im not sure really, he has been learning about cube, cuboid, sphere and other 3D shapes, mentioning words like faces and vertices and then just asked about the name of a 3D Pentagon and Octagon.

I dont understand myself so im not sure what to tell him?
Would I be right telling him a 3D pentagon is a pentagonal prism or a dodecahdron (how do you pronounce it )

*dodecahedron

'Tis a pentagonal prism and octagonal prism (ex Y2 teacher). Dodecahedron has 12 faces! Pentagonal prism has 7 faces.

both are correct.

A pentagonal prism is what my DTDs would call a "sticky out" pentagon. I would say this is what he is thinking of.

A dodecahedron is like a sphere but with pentagons as faces like this. A football is a tangiable object which is similar (although I think a football is typically hexagons and/or pentagons and/or octagons.

You could show him both pictures...and give him the names of both (pic to prisms is in my previous link)

an octahedron is made from 8 equilateral triangles.(looks like a flying saucer.)

An octahedron is 2 squarebased pyramids stuck together! (joined at the squares)

Thanks to you all for replying, I'm going to show him both, then he has all the answers

Has he got magnetix/geomag? He could make the shapes then.

DTDs loved doing this when they were leaning geometry last year.

Alternatively this website has templates you could cut out and stick together to make models. Sounds as though he has enough interest to be fascinated by this...mix it with a pair of scissors and a splodge of glue and he sounds as though he would be in heaven