How to tie a knot on a garbage bag on Shabbat

It's Shabbat morning and you've just noticed that the garbage can is full. You decide to take the bag down to the trash, and you're about to tie the handles together when... Stop!

Are you permitted to tie that knot on Shabbat?

It depends what knot you want to tie. The double-knot you probably use on weekdays is prohibited on Shabbat, as it will remain permanently in place. A bow knot such as the one you use to tie shoelaces is also prohibited, however. On shoes, a bow knot may be made because you'll be untying the knot within 24 hours. (If you usually slip your shoes off without untying the laces, a bow knot may not be made.)

But if you make a bow knot on a garbage bag, chances are it will remain intact for the long-term, and you certainly don't have any intention of untying it yourself. Therefore, the only type of knot you can make is a single-knot, even though (and precisely because!) it may come untied.

In summary –

It is only permissible to tie a garbage bag with a single-knot. Both a bow knot and a double-knot are forbidden.