Poke a hole in both ends of an egg with a needle. Wiggle the needle back and forth to enlarge one of the holes to the size of a pea.
Blow into the small hole, forcing the egg's contents out of the larger one.
Set the egg aside to dry overnight.
Place a piece of clear tape over the smaller hole to plug it.
Pour liquid latex into the larger hole, filling the egg halfway. Slush the latex around the egg to evenly coat the inside, then pour it back out.
Allow the latex to dry for several hours. The larger hole must remain open for this or the latex will not dry.
Repeat this process with two additional coats of latex.
Mix the two components of soft, self-expanding polyfoam in a mixing cup. The mixing ratios will be provided on the polyfoam's instructions. Wear rubber gloves and use a craft stick to mix the components.
Pour the mixed polyfoam into the egg, filling it one-third of the way.
Allow the polyfoam to expand, filling the egg. It will cure in 30 minutes.
Clean off any polyfoam that has squirted out of the hole during expansion.
Break the eggshell to remove the latex egg.
Paint the egg if desired. Use paints that are designed to adhere to latex, available at your local art store.