I like the "pointless" researches like bubble gun and donuts gave me a ice laugh.
A tip for those having problems with earth:
Select earth and then click back and then go to the first world and play it. Repeat as long as you like. THe money and technology you gain goes to the next world you select.
A quick guide:
1: Upgrade Colt's damage and reload to the max
2: Upgrade Scavengers 1lvl
3: Upgrade Builder 1 lvl, this will save you a lot of money through the gaem
4: Save money for the scorpion, upgrade first damage and reload then clip
5: Upgrade Scavengers to the max
6: Upgrade Assistant until you have enough money to spare for next weapon. I upgraded my assisstant to lvl 4 before buying the steyr. The fully upgraded scorpion still stings at wave 33.
I hope this helps!
Have fun!
Do you need to wait out a combo to get the 100,000 badge? Mine is over 700,000. 1k for every 2-3 blinks of the score on the screen. I shouldn't have spent all the money I got for getting 75k.