What's on your finger?

Jake took Isaac with him to a youth day event at the Oxford Church of Christ last weekend. They had to get up at 6am which is really early for Isaac, but he was still excited.  As Jake was getting his keys and preparing to leave in the dim light of the early morning, Isaac extended his thumb and told Jake to touch it. When Jake touched it he asked, "Why is your thumb sticky?" to which Isaac replied, "It has a boogie on it.  It's from my nose.  Get it daddy."