Many foods do not come in natural packaging that is as useful and versatile as its content. Eggs are an exception. So, the next time you buy a carton of eggs, be sure to hold onto the eggshells after you are finished cooking with them.

• Unclog your kitchen sink – Leave a few pieces of egg shells in your sink strainer, which helps trap additional solids.
• Clean your teapot – Mix together crushed egg shells. Dish soap and hot water. Add to teapot and shake vigorously. Let it sit overnight and rinse out.
• Fertilise your soil – Eggs add valuable minerals and calcium to the soil. Add used eggshells to your compost pile.
• Start seedlings – Add potato soil to an egg shell half and keep it upright in an egg carton. Use it to sprout seedlings.
• Keep slugs (and other pests away) – Crush up eggshells to sprinkle a border around your plants to keep snails and slugs away.
• Make sidewalk chalk
  – You will need – 4-5 eggshells (washed + dried), 1 teaspoon flour and 1 teaspoon hot tap water (Optional : food colouring).
  – Step 1 – Place egg shells in a bowl and crush them into a fine powder. Discard any large pieces.
  – Step 2 – In another bowl, add flour, hot water and 2 tablespoon of eggshell powder. Mix into paste (optional : add food colouring). Mix it up.
  – Step 3 – Shape into shape of chalk and roll tightly in a paper towel or wax paper.
  – Step 4 – Allow for it to dry for 3 days. Enjoy.
• Make your coffee taste better – (Only use eggs shells from hard boiled eggs) Add crushed egg shells to ground coffee before brewing. This will make your coffee taste less bitter.
• Add a youthful flow to your face – Crush clean, dried egg shells with a mortar and pestle. Mix powder with egg white and apply to face. After the face mask dries, rinse off.
• Give your pet a calcium boost – In a 250 degree (F) oven, dry eggs shells for 30 minutes. Place in plastic zipper bag and crush with a rolling pin into a fine powder. Place powder in to your dog’s food on a regular basis.
• Make painted Easter eggs – Poke holes on both ends of a raw egg and blow out the egg yolk into a bowl. Rinse out egg and bake at low heat. Once eggs are baked, paint with watercolours.

Reference : https://thesecretyumiverse.wonderhowto.com/how-to/10-unusual-uses-for-discarded-eggshells-0135541/