The gemorah in Shabbos 48a brings two seemingly contradictory statements. First the gemorah says that it is permitted to stuff feathers into a pillow on Shabbos. The gemorah then brings that it is forbidden. To answer the contradiction the gemorah answers that to return the feathers to the pillow from which they fell is permitted, but to put them into a different or a new pillow is forbidden. Tosafos adds that if the normal procedure is to tie a knot upon filling the pillow, even to the original pillow is forbidden.
The gemorah can be applied to many cases. To inflate a ball or an inflatable mattress for the first time after opening the package is forbidden. But many poskim permit to reinflate them after the air has come out. However, to inflate a balloon, even if not for the first time, is forbidden. This is because it is normal to tie a balloon upon being filled with air. Another application of this halacha is to insert shoelaces into a pair of shoes for the first time is forbidden. But if the laces came out, it is permitted to put them back in.
Something which normal to be periodically taken out and then put back in may be placed even for the first time. For example, one can put a belt into the loops of a new pair of pants.

Rabbi Tzvi Aryeh Hyman, Mir Yeshiva
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