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prompter
How do you make a portal in Minecraft?
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2af4ff3d-8b62-4610-9533-1ee139f431a7
assistant
To create a nether portal in Minecraft, you will need at minimum 10 blocks of obsidian and some way to make a fire, the easiest way would be using a flint and steel.
 
Place the portal in like the diagram bellow, where X are any blocks and O are the obsidian blocks, and the portal is standing up vertically.

XOOX
O    O
O    O
O    O
XOOX
 
Then use a flint and steel on the inside ring of the portal and it will activate it.
818b9541-7d9b-4f63-9d62-dd26cb6f063f
b8a2eeb8-1353-4f23-8039-d7c39cced5aa
prompter
Can I make it bigger?
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818b9541-7d9b-4f63-9d62-dd26cb6f063f
assistant
As of snapshot 13w37a, you can now create portals of any size between 2x3 and 21x21. This is a feature in Minecraft 1.7.