When we assign vocabulary activities to students at the beginning of a unit (with the best of intentions), we may be working against ourselves. Imagine someone trying to describe the surface of a newly discovered planet to you. . . in a foreign language. Not only do you not understand the words, you also have no schema to build an understanding of what those words mean. Perhaps that's what our students feel like when they are defining new vocabulary words that will describe phenomena or science concepts for which they have no schema.
I would like to propose an alternative method--one that emphasizes the importance of vocabulary, but only as students are able to understand and apply the meanings of the words. In 7th grade science at TK, we recently started a unit on waves. Our first two days of the unit involved immersing the students in the experience of waves. We demonstrated the stadium wave, and we explored waves on a virtual oscilloscope so that student had experiences and schema to build upon.
Then we were ready to introduce the important vocabulary that scientist use to describe waves. At this point, we may have been describing that planet in a foreign language, but we had taken the students to the planet first so they could associate those foreign words (crest, trough, amplitude, etc.) with the things they had experienced.
The next time you're tempted to start a unit by assigning vocabulary to "get students ready to learn about the topic," consider giving them an experience first and then coming back to add the vocabulary to the schema they build from the experience.