Of course, the danger is 'explosions'.
I make sure I 'burp' my bottles as shown in the below videos.
In summer, I often forgo the sealed bottle, and do the second ferment in a more open bottle. You can see two of my 'closed' ferments on the left, and a more 'open' bottle on the right. I still get a slight fizz in the open bottle. Another option is a bottle with a cloth on the top (to keep out bugs, but let the gas out).
If you feel your bottles are too carbonated to burp, then put them in the fridge to slow down the fermenting, if you are still worried after that - open them outside!
When I travel across town to teach kefir classes, I bring my kefir bottles in a cooler bag with lots of ice blocks - nobody wants a class that starts with a bang.
The below videos show 'how to burp' your bottles.
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