Materials: (feel free to make substitutions)
Set up a gap for the bridge to the gap cross. (We did just shy of a foot so that a ruler could be used to help with determining the bridge length.)
Let your kids explore how they can use the marshmallows to make a bridge for the gap.
Test their ideas.
Repeat Step 2 and 3 if the first attempt failed. Ask them about other bridges they have seen or pull up pictures of bridges. This activity may take time so don’t fret if there is not a quick solution. Remember there is something to be learned from each bridge that doesn’t work.