This is a short lesson that students can have a lot of fun with and illustrates the scientific method. You will need to buy plastic eggs like the kind they sell around easter. Fill the eggs with 1 to 4 nails (or you can use any other items). Each student gets an egg and is asked to make an educated guess about how many nails are inside. They can take data in a few different ways: by listening to the sound, by observing how the egg rolls on a surface, or by weighing their egg - you can even allow them to weigh the nails.
This activity can be made more challenging by including a mix of other items, such as marbles, coins, or beans.
Students can either check their guess by opening the egg or you can have the eggs sealed and keep a "key" of how many nails are in each. I have mine sealed because the cheap plastic eggs tend to fall apart too easy and ruins the activity. Plus, some students just can't resist checking the contents.
You can substitute boxes instead, if you can get a uniform set of boxes that would be ideal - like jewelry boxes. One thing I like to collect, is altoid or mint tins, which can also be used for this kind of activity. It may take a long while to collect the tins, but they are very useful for storing all kinds of objects, such as dissecting pins, thumb tacks..etc. Medicine bottles can also be used, but check with your school, even over the counter drugs (empty containers) may cause issues.
Extension activity: Ask students to create their own mystery egg or box to share with the class the next day.