Squeezy bottle of red sauce to squirt all over your guests at the next BBQ.
Thankfully you wont need to offer to launder their clothes, as its only red string.
This charming practical joke is very effective. The bottle looks convincing so when you squirt it on your friends shirt they will be genuinely shocked.
What comes out of the bottle is a length of harmless red cord but if you move fast it really does make you think you are getting covered in ketchup.