Blow into a re-sealable bag, like a Ziplock sandwich bag, and then seal it. Have the child cover one ear with their hand. Place the inflated bag next to their other ear and gently tap it with the eraser part of the pencil.
Next fill the bag with water - not too much - and seal it. Repeat the experiment by tapping on the bag with the pencil. Talk about how this sounds different than when the bag is filled with air.
Next, instead of using the bag, place the block by the child's ear. Gently tap the block with the pencil. It sounds much different! Have the child talk about the differences between the three sounds.
Explain that sound waves travel easier through solid objects and that makes the sound better. Great project to begin introducing small children to science!