Stir 1/8 cup borax into 500 ml warm water. It's okay if some borax remains undissolved. Allow solution to cool to room temperature.
In a separate container, stir 2 spoonfuls of glue into 3 spoonfuls of water. White glue will give you opaque snot (more booger than snot); glue gel will produce you translucent slimy snot.
Stir a couple of drops of food coloring into the glue mixture.
Add a spoonful of the borax solution to the glue mixture. Stir (if in a bowl) or squish (if in a baggie).
Have fun with your snot!
Elmer's glue is a non-toxic, water-based white glue or a non-toxic blue or clear gel. Another brand may be substituted.
A little yellow food coloring could be added for that extra sickly touch.
Don't eat or inhale the snot. Borax isn't particularly toxic, but isn't good for you either.
Borax is found in the household cleaner or laundry section of the store. It's commonly used as a detergent booster.
Wash your hands after playing with your snot.