This is a great prank!
Step 1: Materials
Orbeez or something really slimy
Step 2: Construct the Box
Cut 5 equal squares out of poster board. Tape them together as a box. Cut out another square and set aside.
Step 3: Finish Box
Fill your box with orbeez. Join 4 twist ties together. Put the last square and tie with twist tie. You can wrap the box now. Give the box to someone who is easily grossed out. Tell them to close their eyes and put their hands in the box.