Instructables
Picture of Home made air lock
 How to make a simple air lock for home-brewing.

 
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Step 3: Construction

 Push the short end of the straw about 1cm through the bottle lid and seal around it with your sealant or blu-tack.
Push the other end of the straw through the canister lid so that when it is put on the canister it isn't touching the bottom otherwise it may block causing a build up of CO2 that would make your home brew go of like a shaken soft drink.
Finally fill the canister with water leaving some room at the top but ensuring that the end of the straw is covered.
quoct21 month ago

what will you do with the second hole of blue cap ? thanks

-CCCP-3 months ago

Homebrewing should be illegal!

rimar20004 years ago
What is an air lock? I don't speak English, pardon.
98cmcmaster (author)  rimar20004 years ago
 Hi when you are making home brew you need to allow the carbon dioxide gas produced by the yeast to escape but you also need to stop harmful airborne bacteria and mould spores from getting to the brew. An airlock allows the carbon dioxide to bubble out through the water but doesn't allow the air into the brew. Hope this helps feel free to ask for more clarification.
Thanks, very much. Good explanation.