3. what is crossing over? when does it occur during meiosis? why did you have difficulty modeling this element of meiosis using your “chromosomes? ”

Crossing over happens in Meiosis 1 only. In Prophase 1 a cells chromatin (chromatin = chromosomes that have not condensed yet) condense and pair up forming homologous chromosomes (paired = XX (2 chromosomes together)). When this happens segments/ alleles of the chromosomes pairing up swap over. This creates genetic diversity as each chromosome is different, it has parts from its pair. This leaves every chromosome unique and individual.

I hope this helps, sorry some of the vocab is rather technical. By the way I would suggest watching the 'Crash Course: Meiosis' on YouTube, this really helped me when I was learning this topic. :D 

There would be no evaporation so no water cycle