Scientists use the information they find/learn/acquire along with their own background knowledge to make hypotheses to explain what they're observing. As they acquire more information, those hypotheses can change to incorporate the new information. Scientific knowledge constantly changes.
This activity demonstrates this process, in a classroom-friendly, time-friendly manner. It's a new activity to me, but looks like a keeper. Exact instructions can be found here, but here's a synopsis.
A long sentence - The big fat red dog walked into the little white house on the prarie carrying a bone and ate his bowl of turnips - is printed and cut into individual words, which are placed in an envelope.
Students select 5 words from the envelope. Using the words they've selected, they hypothesize what the story is about.
An additional 5 words are selected. Using all 10 words, the students tweak their hypothesis to incorporate the new information.
Another 5 words are selected. Using all 15 words, the students refine their hypotheses some more.
Finally, all the remaining words are removed from the envelope and a final hypothesis is created.
Even after all the groups have all the information, they will likely have variations in their hypotheses. Why? What does this tell us about science?
Additionally, Liz LaRosa blogged about her use of the activity and has been generous enough to share her worksheets for use with this activity. If you haven't checked out www.middleschoolscience.com, you really need to - it's loaded with great stuff!