Trigger if text variable contains

I have a text entry box that changes a text variable. I want the state on an object to change when/if this variable CONTAINS a certain word i.e. the learner could write a longer sentence but all that matters is that this certain word is there. Is this possible? I'm trying to avoid having to think up / input all the possible variations as this seems impossible. 

For example - 

If the learner writes a sentence that contains the word tree, a picture of a tree that is hidden becomes normal. Regardless of if they have just written 'tree' or 'the tree has leaves' and so on.

Thanks!