This challenge refers to travelling as green as possible for a specific time period. Some transportation methods like cars, contribute to climate change by burning fossil fuels, releasing carbon dioxide and other greenhouse gases into the atmosphere, causing global temperatures to rise. 

The allowed traveling methods include: traveling by walking, biking, taking subways, etc. 

This challenge refers to not using single-use plastics for a specific time period. When single-use plastics are disposed of improperly, they often end up in landfills or oceans, where they can take hundreds of years to decompose, releasing more greenhouse gases in the process. Plastic pollution also harms wildlife and ecosystems, disrupting the balance of the environment. 

However, as plastics slowly turn into something we cannot live without, all we can do now is to prefer using reusable plastics instead of single-use plastics. 

This challenge refers to not eating meat for a specific time period. Meat production contributes to global warming primarily through the release of greenhouse gases such as methane and nitrous oxide. Livestock farming, particularly cattle, produces methane during digestion and manure decomposition. 

However, with no meat, the "Vegan challenge" might be a bit too hard for most people, therefore the "Vegetarian challenge" might be more suitable for the majority.