A Dan Meyer blog post and  video (36 seconds) to perplex and engage students.

Here’s one possible way to use it:

• Stop it at 8 secs. in and ask students what they notice.
• Resume playing and stop it at 12 secs. and ask how much 20 marbles will weigh. Ask them to share with their neighbors and then have a few share their reasoning with the whole class.
• Resume playing and stop it at 14 secs. Ask them what happened? What could explain it?
• Repeat with 30 marbles (stop at 17 secs) and 40 marbles (stop at 23 secs.) so students can see the weight change of adding 10 more marbles.
• After the whole video, ask students to explain what happened in their own words. Then have students to graph the data and put the numbers in a table.