Make a mycelium starter

What can we use instead of mushroom spawn or coffee grounds/cardboard if those are hard to find?

If mushroom spawn is hard to find, buy pre-sterilized grain or sawdust spawn from a reputable supplier and use it in the same one part spawn to five parts substrate ratio when you "Add mushroom spawn to the bowl"; if coffee grounds or cardboard are unavailable, pasteurized straw or sterilized hardwood sawdust can be used as the substrate instead.

What should we do if nothing seems to grow or the jar looks slimy or green instead of white?

If you see green or black spots, slime, or no white threadlike mycelium after placing the closed container in a dark warm spot, discard the contaminated batch, thoroughly clean your tools and container, ensure the pasteurized substrate was fully cooled before adding spawn, and try again using clean gloves and sterile technique to prevent contamination.

How can we adapt the activity for younger children or older kids who want more challenge?

For younger children have the adult handle the hot pasteurization and making ventilation holes while the child writes the date with the permanent marker, spoons the mixed material into the labeled container, and lightly mists the surface under supervision, whereas older kids can measure volumes for the one-to-five spawn-to-substrate ratio, experiment with different substrates, and keep detailed growth records.

How can we extend or personalize the project after the mycelium starts to grow?

To extend the activity, decorate and label multiple containers, keep a photo log of mycelium progress from when you "Place the closed container in a dark warm spot", compare coffee-ground versus cardboard results in parallel jars, or tweak humidity by adjusting lid holes and misting frequency to see how colonization speed changes.