The secret to the underwear is in the layers. The Spanx-y outer fabric is leak-proof: In an experimental trial, I was able to pour a quarter-cup of water into the underwear before the liquid started dripping out. That is the same amount of water I pour into my body in a day. (I'm severely dehydrated.)

I should add that Thinx only claims to replace panty liners. You can use these in conjunction with your tampon. But I was put on this earth to test limits, so I threw my tampons in the TRASH.

Then I fished them back out, because I only bought one pair of Thinx and would definitely need tampons for the rest of the week.