How to fill water balloons with viscous substances?
Use a funnel to put mud into a balloon. Another good idea is to mix corn starch and water. It makes a sticky substance that is not quite a solid, but not really a liquid, either. Spoon some cornstarch into a balloon and then fill it the rest of the way with water. Use about the same amount of water as cornstarch and leave a little empty space at the top. Once you fill the balloons, let them sit a while, and try not to shake them. Happy pranking!