Milk carton blocks for anyone that has never done them: open them up completelly and fill with water and bleach - I usually let them sit over night and then dump the next morning and let them sit for 2-3 days to be completely air dried. Then cut off the parts that fold. Do this to two cartons. Pinch one carton by pushing in slightly on all 4 sides and insert it into another carton. You will need to push on the top to get it all the way down. If you want you can cover the blocks with mac tac or paper or whatever but I find that a waste of money because the kids are quite content to use the cartons as is. When they get squishy, pull them apart and sometimes you can use the two squishy parts as the inserts into two new cartons. The are great for piling up and using a stuffed toy to play humpty dumpty. The 2L cartons work the best because they have a larger flat side to them but I do it with the 1L too. The smaller pint etc can be done but because the block isn't perfectly flat when made they don't stack too well and can frustrate so stick with the bigger ones.