Last fall we left the bin of Duplos out on the porch for a few weeks and they became dirty and a few spiders took up residence in the blocks. We had fun filling up the sandbox with soapy water and we spent a morning washing duplos in the backyard. Ever since then, Noah will occasionally look through the Duplos and say, "hmmm..." as he inspects them. A few days ago, he dragged the bin to the kitchen sink and said "these are dirty." He pushed his step stool up to the sink, climbed up and asked me to put bubbles in the sink. He spent quite a bit of time washing before he declared that all his blocks were clean and ready to use again.