If students can't write about a concept in their own words, they just don't know it. End of story.
But having students simply write about a topic can be pretty boring, in both product and process. One of the ways I've found to spice up student writing as evidence of understanding is to use the RAFT strategy. RAFT stands for Role, Audience, Format, and Topic, and it's a strategy that's been around for a while (I first learned about it way back in 1999 at a conference). However, I find it's pretty darn effective at getting students engaged in their writing, as well as having students synthesize concepts into a coherent whole. In other words, it's good at getting the individual "planks" of random science stuff bound together into one coherent "raft." Below is the RAFT I wrote for my Biology classes yesterday:
Want to give the RAFT strategy a try? You can check out the resources below:
RAFT Writing Template
RAFT Examples for Different Subject Areas
Lots of RAFT Examples